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Such being the structure of teeth and their three component parts, it is evident from our verbal sketch, that they deserve a far higher character than that of mere inorganic exudations. Without entering into the history of their development, by the consolidation of the soft tissues forming the pulp of the tooth, the knowledge of the fact is of the utmost importance in interpreting the phenomena of disease, and has already led to great improvements in

dental surgery.

The varieties of dental structure produced by differences in the arrangement and relative amount of these three components are in themselves very numerous; and they are vastly increased through the diversities exhibited by the minuter characters of each tissue. To some of these we have already alluded; and instead of formally detailing others, we will adduce a few examples, by way of proof, of the assistance which microscopic research has already rendered to the Naturalist.

There have been few questions in palæontology more fruitful of discussion, than that of the precise zoological affinities and habits of life of the great extinct Megatheroid quadrupeds of South America. They present the strange phenomenon of a skeleton more closely resembling that of the sloths than any other; not merely expanded to a colossal size, but developed with a massiveness unparalleled in any other animals of similar dimensions. The feet, on the other hand, were constructed rather upon the plan of those of the ant-eaters and armadilloes; being obviously capable, in virtue of their long, curved, sharp-edged claws, of being used as efficient instruments for digging or excavating the soil. In consideration of these peculiarities of conformation, as long as attention was not paid to the characters furnished by the teeth, an opinion was entertained by many distinguished palæontologists, that these strange beasts must have burrowed in the ground, like the pigmy armadilloes of the present time, and have fed upon the roots which they there met with. From the examination of the minute structure of their teeth, however, ample evidence was obtained by Professor Owen, that the food of the Megatheroids must have been of the same description with that of the existing sloths; that is, it must have consisted of vegetable matter capable of being easily reduced to a pulpy state, such as the leaves and young shoots of trees : for the teeth are entirely destitute of enamel; without which it is impossible that tough fibrous roots could be ground down. The greater part of their substance is made up of coarse vascular dentine; the exterior being composed of vascular cementum, and the harder non-vascular dentine forming only a hollow cylinder between the two. The conformation of the skeleton having been re-investigated by Professor Owen under the guidance of this most important indication, he succeeded in obtaining a most satisfactory solution to the problem of their mode of existence. They prove to be Sloths in all their essential characters—but adapted to live upon the ground, instead of climbing trees; which could not have sustained their enormous weight, even had it been possible to endow them with the climbing power. Not only were their limbs expanded to a colossal bulk, but, from its size and strength, their tail also was obviously designed for an organ of support; forming a firm tripod with the hind legs. Upon this tripod we may conceive the gigantic Mylodon or Megatherium to have reared itself, after having excavated with its trenchant claws the earth around the roots of the trees on whose leaves its hungry eyes were fixed; thus, raising itself upright, and placing its fore-feet against the trunk, it would sway the tree to and fro, until it had brought it down, and so provided itself in its foliage and tender shoots with a supply of food.

The 'Odontography of Professor Owen will afford us another example of the value of the microscope in determining the nature of a doubtful fossil. This work contains the results of the laborious and accurate observations of its distinguished author upon almost every principal tribe of the three classes of vertebrata in which teeth are present; and must constitute the foundation of all future researches on this subject. In the case in question, a few scattered teeth were almost the only indications of animal life throughout an extensive stratum : and they happened not to be characterised by any peculiarity of external form—so that their real nature would have remained doubtful, unless the microscope had enlightened us on their internal structure. Some years ago, certain detached teeth were found in the keuper-sandstone of Wirtemberg, and were described

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by Professor Jaeger as the remains of a gigantic Saurian reptile, to which he gave the name of Mastodonsaurus. Other fossil fragments of jaws and teeth from the same formation were described under the name of Phytosaurus. A third remarkable and characteristic fossil discovered in the keuper-sandstone, consisted of a fragment of a cranium having certain peculiarities of the Batrachian reptiles; and on this fossil Professor Jaeger founded his species called 'Salamandroides giganteus. In a sandstone deposit in Warwickshire again, certain teeth or fragments of teeth, at first supposed to be of saurian character, had also been discovered. Whether this sandstone was the equivalent of the keuper or of the bunter division of the new red standstone formation, as developed in Germany, was an important question. Accordingly, it became also of importance to determine whether the Wirtemberg and the Warwickshire fossil teeth were of identical or of allied species, or whether they were altogether dissimilar. Now the external forms of the teeth of reptiles are seldom so characteristic as in Mammals, whose teeth are much more adapted to their several kinds of food. It is not at all surprising, therefore, that Professor Owen was unable to pronounce upon this question, from the comparison of their external aspect alone. But, on procuring a thin section of one of the Wirtemberg teeth, and placing it under the microscope, a most wonderful and complicated structure presented itself to his astonished view. Of this structure we almost despair of conveying an idea by words alone. We will, however, try with the assistance of some of the forms which connect it with the ordinary types of dental structure. Our readers must first picture to themselves a hollow cylinder of dentine, covered externally by cementum, and enclosing the soft pulp of the tooth ; and then imagine the cylindrical wall to be thrown into a regular series of wavy folds arranged vertically: they will thus understand the formation of the lower part of the tooth of the extinct Icthyosaurus-and also that of the tooth of the Lepidosteus or Bony Pike, one of the few sauroid fish still existing. But let them further suppose these folds to be deepened, so as to become a series of plaits, all directed towards the axis of the cylinder, and in contact with each other at their sides; and afterwards, suppose the plaits themselves to be thrown into secondary folds and convolutions, so as to present very much the aspect of the surface of the brain ; they will then have some idea of the extraordinary structure of this tooth, the original possessor of which has since received the very expressive designation of Labyrinthodon, the former name of Mastodonsaurus being no longer appropriate. It was only after making sundry sections of this tooth in various directions, and frequently comparing them with numerous examples of the teeth of Saurians, Batrachians, and other animals, that Professor Owen at length comprehended the cerebriform convolutions by wbich every portion of the tooth of this most singular reptile of the keuper-sandstone is so remarkably distinguished. Once fully comprehended, however, all this complexity was found to be apparent only: for each wavy

fold has on one side of it an extension of the central pulp-cavity, from which its dentinal tubes proceed; whilst its other border properly forms part of the external surface of the tooth, carried in by this extraordinary duplication towards its centre, and thus everywhere in its usual relation with the cemental layer.

The next step was, of course, to make a similar examination of the teeth from the Warwickshire sandstone. The result left no doubt but that they belonged to the same genus with the German specimens; and this, in connection with other evidence, sufficiently proved the equivalence of the German and English deposits. A most interesting zoological problem now came under consideration ; -what was the nature of the animal to which these teeth belonged ? They had been referred, from external characters, to the Saurian order ; but these characters were by no means conclusive, owing to the general similarity of form which prevails through that entire class. As the nearest approaches to the peculiar internal structure of the teeth are presented by Fish-lizards, and Lizard-like fish, it might be reasonably expected that the Labyrinthodon would combine with its reptilian characters an affinity with fish. This turned out to be the case on an examination of the fragments of the skull of the Warwickshire fossils, with which the teeth remained in connection; and the analogy was confirmed by comparing them with the Batrachian remains in the keuper-sandstone. All things considered, there can be no question, we think, but that the Labyrinthodon was in reality a gigantic Batrachian or frog-like animal, five or six feet in length, with some peculiar affinities to fishes, and a certain mixture also of Crocodilian characters.

Now, it happens that, in various localities, where beds of the new red-sandstone formation have been exposed, and especially in the Stourton quarries, near Liverpool, foot-prints are found, which afford unmistakeable evidence of the former existence of an air-breathing animal, whose posterior extremities must have been of most disproportionate size, and its foot of a singular hand-like shape. The provisional name of Cheirotherium had been bestowed upon this unknown quadruped, and more than one palæontologist suggested that the creature must have been a Batrachian reptile. But it was objected that the gigantic dimensions of the hypothetical frog were without a precedent; and that its feet were unlike those of any existing Batrachian, or, indeed, of any reptile whatever. We have now seen, however, that in beds of the very same formation, there have been found the teeth and bones of a gigantic Batrachian reptile, whose dimensions and proportions would agree with the foot-prints in question; and which differs from all other Batrachia, -indeed from all other reptiles,-in the structure of its teeth, and therefore (it may fairly be inferred) also in the structure of its extremities. It appears to us, therefore, that a very satisfactory case is made out by Professor Owen in favour of the identity of the Labyrinthodon and the Cheirotherium; and we cite it, for the purpose of observing that the most important link in the chain of evidence was furnished by microscopic examination of the teeth.

Though we have dwelt so long upon this example, before leaving it we wish to notice a curious fact connected with it. Very shortly after the publication of the second part of Professor Owen's Odontography, containing his beautiful delineations of the dental structure of this fossil, a pockethandkerchief was made in Manchester, with an enlarged copy of the section of the Labyrinthodon tooth for its pattern! It would be well, we think, if our manufacturers had recourse more frequently to Nature for suggestions. The sections of the spines of the Echinus, whose remarkable

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