-an

cannot with propriety be referred to the Nudibranches,order so remarkable for the beauty and variety of the branchial appendages with which the species are adorned. "The mollusks now under consideration are, on the contrary, distinguished by the extreme simplicity of external form, and by the absence of any specialized breathing organs. It would, therefore, be more in conformity with the views. on which the existing orders of Gasteropoda were established by Cuvier, to consider this group as forming a separate order, characterised by the absence of specialized branchiæ; and as the function of respiration is entirely performed by the skin, we propose to call this order PELLIBRANCHIATA. It includes the genera Elysia or Actæon, Limapontia or Chalidis, Acteonina, and Cenia: and it is allied both to the Nudibranches and Inferobranches, by genera which may be deemed almost inosculating. You must refer to the original essay for the anatomical peculiarities that distinguish the order.*

Class ACEPHALA.

It is now agreed that the Heterobranches + or Tunicata. ought to be removed from this to form a separate class; but there is not yet such an unanimity amongst conchologists as to the rank of the Brachiopoda. Cuvier considered them entitled to a classical rank from the structure of their breathing organs and the possession of their curiously ciliated arms; but Professor Owen regards them as rather an order in their class. "In all essential points," he says, "the Brachiopoda closely correspond with the acephalous Mollusca, and I consider them as being intermediate to the Lamellibranchiata and Tunicate orders; not, however, possessing, so far as they are at present known, distinctive characters of sufficient importance to justify their being regarded as a distinct class of Mollusks, but forming a separate group of equal value with the Lamellibranchiata." Deshayes, a name of great authority in this point, has adopted this conclusion; and he gives us his views in a table which, it seems to me, is well worth transcribing in this place.

First sub-Class. BRACHIOPODA, or

POLYMYARIA.

CONCHIFERA.

(1st sub-Order: Valves articulated.

2nd sub-Order: Valves free.

On a proposed new order of Gasteropodous Mollusca, by Joshua Alder and Albany Hancock in Ann. and Mag. N. Hist. Ser. 2. i. 401, &c.

66

+ "A name without one single quality to recommend it.”—MACLEAY. Zool. Trans. Lond. i. 159.

Shell adherent. Thecidea.
** Valves free.

Te

4. LINGULIDE. A long tendinous pedicle fixed to the beaks. Lingula.

5. ORBICULIDE. Pedicle short, by the middle of the lower valve. Orbicula.

6. CRANIADE. Shell adherent without ligament. Calceola, Crania.†

DIMYARIA.

7. TUBICOLIDæ.

Order I.

* Shell regular.

Shell incrusted in whole, in part, or free in a tube which is either free or enclosed. Aspergillum, Clavagella, Fistularia. 8. PHOLADARIA. Shell free or within a tube, without ligament; with apophyses under the beak. Septaria, Teredo, Teredina, Pholas. 9. SOLENACEA. Shell gaping at the ends; ligament external; hinge simple or with one or two hooked teeth. Solemya, Glycimera, Pholadomya, Solen, Panopæa.

10. MYARIÆ. Shell inequivalve; ligament internal

+ Mr. J. E. Gray has recently published a very elaborate classification of the Brachiopoda in the Ann. and Mag. N. Hist. Šer. 2. ii. 435.

upon a perpendicular tooth. Mya, Corbula,

Pandora. 11. OSTÉODESMES. Shell inequivalve; ligament internal; a free ossicle at the hinge.

Osteo

desma, Periploma, Anatina, Thracia. 12. MACTRACEA. Shell equivalve, closed or slightly gaping; ligament internal with or without an external ligament. Lutraria, Mactra, Mesodesma, Crassatella, Erycina, Amphidesma. 13. PETRICOLIDE. Shell free or boring, not enclosed in a tube, gaping, scarcely regular, ligament external. Saxicava, Petricola, Venerupis, Hyatella, Byssomia.

14. TELLINIDIDE. Shell free, regular: two cardinal teeth at most in each valve, sometimes lateral teeth; ligament external, on the lesser side of the shell. Sanguinolaria, Psammobia, Tellina, Donax.

15. LUCINIDE. Shell free, regular, closed; ligament external or subinternal; hinge variable; muscular impressions very large. Corbis, Lucina, Ungulina.

16. CYCLADE. Shell fluviatile; hinge simple or with lateral teeth, or with lateral and hinge teeth; ligament external. Iridina, Cyclas, Cyrena, Galathea.

17. CONCHE. Shell free, regular, closed; cardinal teeth from two to four, sometimes a single lateral tooth; ligament short, external. Cyprina, Venus, Astarte.

18. CARDIACEA. Shell regular; cardinal teeth irregular, one or two lateral teeth; ligament external. Cypricardium, Isocardium, Cardium. 19. TRIDACNADE. Shell regular, truncate, sometimes gaping on one side; ligament external. Tridacna, Hippopus.

**Shell irregular.

20. CAMACEA. Shell irregular, adherent, inequivalve; a large hinge tooth. Chama, Diceras.

21. NAYADES.

Order II.

*Shell regular.

Shell regular, fluviatile, nacred; hinge without teeth or articulation; ligament external.

Unio.

22. CARDITES. Shell regular: one or two oblique

hinge teeth.

Cardita.

23. TRIGONACEA. Shell regular; hinge teeth lamellar, grooved transversely. Trigonia.

24. ARCACEA. Shell close, or gaping inferiorly; hinge multidenticulate, straight or curved, or angular. Nucula, Pectunculus, Arca, Cuculæa. 25. MYTILACEA. Shell regular, fixed by a byssus; hinge simple, edentulous or subdentate; ligament marginal subinternal. Mytilus, Pinna, Avicula.

**Shell irregular.

26. ETHERIDÆ. Shell irregular, adherent; hinge edentulous; ligament in a gutter of the beaks. Etheria.

27. RUDISTES.

Shell irregular, adherent; upper valve operculiform, two prominent muscular impressions in the upper valve; ligament internal. Spherulites, Hippurites, Caprina.

MONOMYARIA.

28. MALLEACEA.

Order I.

Shell free, subinequivalve, with a simple or compound ligament, subinternal in one or several grooves or crenulations. Crenatula, Perna, Inoceramus, Catillus, Gervilia, Malleus, Vulsella.

29. MULLERIA. Shell adherent; hinge edentulous ; ligament external. Mulleria.

30. PECTINIDES. Shell free or adherent, auricled; hinge straight; ligament internal or semi-internal. Lima, Plagiostoma, Pecten, Hinnites, Plicatula, Spondylus.

Order II.

31. OSTRACEA. Shell irregular, foliated, free or adherent; ligament internal or semi-internal. Placuna, Anomia, Ostrea.†

Class-TUNICATA.

I can scarcely consider the alterations which have been made in the classification of Savigny as improvements upon it. The rank of the Tunicata must be granted to be the

Trait. Element. i. 212. This classification was proposed in 1830. See also Deshayes' article "Conchifera" in Cyclop. Anat. and Physiology, i. 695.

lowest in their sub-kingdom. Lamarck and Milne-Edwards go so far, indeed, as to maintain, that they have a nearer relationship to the Polypes than to the Mollusca; and we may safely consider them to be an "osculent" group with Macleay, connecting the Mollusca with the Ascidian Zoophytes. To use Mr. Macleay's words,-" The Tunicata, then, are animals which connect the Acrita, or lowest primary division of the animal kingdom with the Mollusca. From the Mollusca, however, they differ in having an external covering, consisting of an envelope distinctly organised and provided with two apertures, of which one is branchial, the other anal. They also differ from the Mollusca as well in their mantle forming an internal tunic corresponding to the outer covering or test, and provided, like it, with two openings, as in having branchia which occupy all, or at least part of the membranaceous cavity formed by the internal sides of the mantle. From the Acrita they differ in having distinct nervous and generative systems, while their intestinal canal is provided with two openings, both internal."*

This osculent group Mr. Macleay divides as follows, in a rather futile attempt to make the arrangement square with his system of quinary circles :