Animal. Body more or less compressed, orbicular; mantle furnished with a single row of tentacular papillæ, and with small oculiform, pedunculated disks, with regular spaces between them; rudiment of a canaliculated foot, and a byssus; mouth surrounded with fleshy appendages, irregularly ramified.
Shell. Free, regular, thin, solid, equivalve, equilateral, auriculated; summits contiguous; hinge toothless, a ligamentous membrane throughout all its extent, besides a short, thick, ligament, almost altogether internal, and filling a triangular excavation under the summit; a single subcentral muscular impression. Found in almost all seas. Sixty-two species; and, according to Defrance, ninety-eight fossil.
- Pecten maximus.
- P. Jacobæus.
- P. bifrons.
- P. zigzag.
- P. medius.
- P. Latirentii.
- P. pleuronectes.
- P. purpuratus.
- P. pallius.
- P. impricatus.
- P. asperimus.
- P. varius.
- P. glaber.
- P. grisens.
- P. flabellatus.
- P. quadradiatus.
- P. Tranquebaricus.
- P. hybridus.
- P. obliteratus.
- P. Magellanicus.
- P. lineolaris.
- P. nodosus.
- P. pusio.
- P. gibbus.
- P. ornatus.
- P. florens.
- P. opercularis.
- P. tigris.
- P. histrionicus.
- P. turgidus.
- P. aspersus.
- P. plicus.
- P. rastellus.
- P. flagellatus.
- P. flavidulus.
- P. Japanicus.
- P. radulus.
- P. pes-felis.
- P. sauciatus.
- P. aurantius.
- P. sinuosus.
- P. virgo.
- P. Isabella.
- P. flexuosa.
- P. inflexus.
- P. miniaceous.
- P. lividus.
- P. sulcatus.
- P. unicolor.
- P. distans.
- P. irradians.
- P. dispar.
- P. Islandicus.
- P. sulphureus.
- P. pellucidus.
- P. sanguineous.
- P. senatorius.
- P. lunatus.
- P. lineatus.
- · · · · ·
- P. dislocatus.*
- P. concentricus.*
- P. monotimeris.*
- P. Latiauratus*
- P. Pealii.*
4. Genus Plagiostoma. Pl. [IX].
Animal. Entirely unknown.
Shell. Moderately thick, regular, free, subequivalve, subauriculated, the two valves nearly equally dilated, both provided with a distinct summit reflexed to the middle of a plane surface, with a large triangular slope in the middle; articulation transverse, straight, and by two distant lateral condylæ. Ten species. Fossils only.
5. Genus Plicatula. Pl. [IX].
Animal. Unknown.
Shell. Solid, adhering, subirregular, inauriculated, inequivalve, pointed at the summit, rounded and subplicated posteriorly; inferior valve without heel; hinge cephalic, longitudinal, provided upon each valve with two strong teeth, entering in corresponding cavities; ligament altogether internal and inserted in a median cavity. Inhabits the American seas. Five species.
- Plicatula ramosa.
- P. depressa.
- P. cristata.
- P. Australis.
- P. reniformis.