Part 9

14. Chamacea (plates, fig. 153 to 155), inequivalve, irregular, foliaceous, attached; containing the genera Diceras, Chama, and Etheria.

Second Order, Conchifera Monomyaria.

Having one adductor muscle, and therefore only one impression in each valve. They are separated into the following families:--

1. Tridacnacea (plates, fig. 156 & 157), transverse, equivalve, with an elongated muscular impression, near the centre of the ventral margin; margin undulated at the termination of the radiated large ribs. The genera Tridacna and Hippopus are included.

2. Mytilacea (plates, fig. 158 to 162), generally regular, with the hinge linear, without teeth, occupying the greater part of the dorsal margin. This family includes the genera Modiola, Mytilus, Pinna.

3. Malleacea (plates, fig. 163 to 170), shell generally thin, inequivalve, irregular, foliaceous, with the hinge linear. This family contains the genera Crenatula, Perna, Malleus, Avicula, Meleagrina.

4. Pectinides (plates, fig. 171 to 178). The Pectinides are generally regular or nearly so, with the shell solid; the greater part of them are auriculated at the dorsal margin, and generally characterized by ribs radiating from the umbones. The genera are Pedum, Lima, Plagiostoma, Pecten, Plicatula, Spondylus, Podopsis.

5. Ostracea (plates, fig. 180 to 192). The shells of this family are irregular, generally attached and foliaceous. They compose the genera Gryphæa, Ostræa, Vulsella, Placuna, Anomia.

6. Rudistes (plates 193 to 200). This family is composed of a particular association of shells, which appear on one side to be connected with the Ostracea; and on the other to approach the Brachiopoda. They differ from Ostracea in having no hinge or ligament, and only resemble them in their irregularity and foliaceous structure. The following six genera are placed by Lamarck in this family:--Sphærulites, Radiolites, Calceola, Birostrites, Discina, Crania. Of these, Calceola, Discina, and Crania are shewn to belong to the Brachiopoda.

7. Brachiopoda (plates, fig. 201 to 219). The shells of this family are inequivalve, equilateral, and attached to marine bodies by a tendon passing through one of the valves. The animals have, near their mouth, two elongated, ciliated arms, which are spirally rolled when at rest. The following genera are enumerated by Lamarck, Orbicula, Terebratula, Lingula.

MOLLUSCA.

Lamarck applies, or rather restricts, this name to those invertebrated animals, which while they are inarticulate in all their parts, have the head sufficiently advanced at the anterior part of the body to be distinguished; which is not the case with the Conchifera. All the shells are univalve, and are divided into six orders, namely, the PTEROPODA, which have wing-shaped natatory organs or fins, and have light, thin transparent, nearly symmetrical shells; the GASTEROPODA, with the foot not distinguishable from the rest of the body, have patelliform, open, and scarcely spiral shells; the TRACHELIPODA with the foot distinct and attached to the neck of the animal, have spiral, non-symmetrical shells. The CEPHALOPODA, with arms covered by suckers surrounding the head of the animal, have generally symmetrical convolute shells. The Cephalopoda are divided into C. polythalamia, which have the internal cavity divided into chambers by septa, as in the Nautilus; and the C. Monothalamia, which are not so divided, as the Argonauta. The order Heteropoda contains the genus Carinaria alone.

Order Pteropoda.

This order, containing hyaline, symmetrical, non-spiral shells, as above described, is not divided into families, but contains the following genera, Hyalæa, Cleodora, Limacina, Cymbulia; the first of which, although composed of a single piece, resembles a bivalve so nearly, that Linnæus actually placed it in his genus Anomia.

Order Gasteropoda.

With the exception of the genus Bulla and Vitrina, the last of which forms a passage into the next order, the shells contained in this order are patelliform, open, and scarcely spiral. They are divided into the following families:--

1. Phyllidiana (plates, fig. 227 to 231), containing the genera Chiton, Chitonellus, and Patella, the two former of which present the only exception to the statement above made, that all the shells of Mollusca were univalve.

2. Semiphyllidiana (plates, fig. 232 and 233). Of the two genera contained in this family, Pleurobranchus is broad, thin, and slightly spiral at the apex, and Umbrella is flat, circular, with a central apex.

3. Calyptracea (plates, fig. 234 to 246). The patelliform shells of this family, although united by no other general characters, are brought together by the characters of the animals which produce them. The genera are Parmophorus, Emarginula, Siphonaria, Fissurella, Pileopsis, Calyptræa, Crepidula, Ancylus.

4. Bulleana (plates, fig. 247 to 253), contains the genera Bulla and Bullæa.

5. Aplysiacea (plates, fig. 254 and 255). The genera Aplysia and Dolabella are both expanded, somewhat flattened shells, with the apex placed at one extremity, and slightly spiral.

6. Limacinea (fig. 256 to 263). Many of the animals (slugs) are without shells; some, as the Limax, or common garden slug, have a slightly developed calcareous piece, hidden beneath the mantle, and of others the shells are scarcely spiral. The genera included in this family are, Parmacella, Limax, Testacella, Vitrina.

Order Trachelipoda.

All the remaining spiral non-symmetrical shells are arranged in this order, which is divided into the following families:--

1. Colimacea (plates, fig. 264 to 307). With the exception of the few contained in the family of Limacina, which ought not to be separated from this order, the whole of the land-shells are contained in this family, and although it is difficult to notice any one character by which terrestrial shells may be distinguished from others, few at all conversant with the subject are liable to mistake them. There is a general lightness and simplicity of form, which, though not clearly definable, is generally understood. The following distribution of genera by Lamarck, is generally acknowledged to require numerous modifications; the genera are Helix, Carocolla, Anostoma, Helicina, Pupa, Clausilia, Bulinus, Achatina, Succinea, Auricula, Cyclostoma.

2. Lymneana (plates, fig. 308 to 312). The shells of this family are found in fresh water, wells, ditches, and ponds. They are of a light horny structure, and simple form. The genera Planorbis, Physa, and Lymnea are placed in this family by Lamarck.

3. Melaniana (plates, fig. 313 to 317). These are also found in fresh water, principally in rivers; they are thicker than those of the last family; and the greater part of them have elevated spires composed of numerous whorls. This family contains the genera Melania, Melanopsis, Pirena.

4. Peristomata (plates, fig. 318 to 322). These are also fresh-water shells, having opercula, and covered by a smooth green, or greenish-brown epidermis. They differ from the last family in having the peritreme entire. The genera are Valvata, Paludina, and Ampullaria.