Lamarck's Genera of Shells. 39 



anged these shells with the ostrea>, but they differ from them 

 in being free, regular and almost ctjuivalve, and from the pec- 

 tines by their remote beaks, and cardinal pit. They are sea- 

 shells, and generally white. 



Type. Lima squamosa *. (Ostrea Lima. Linn.) 

 Shell oval, depressed, cut off as it \Vere at the fore part ; ribs 

 squamosa, very rough ; hinge oblique ; margin plicate. 



American Ocean. 6 recent species, and 5 fossil. PI. 11. Fig. 87. 



3. Plagiostoma t- 



Shell subequivalve, free, subauriculated ; cardinal base 

 transverse, straight. Beaks rather remote, their inner sides ex- 

 tending into transverse, flattened, external facets, one straight, 

 the other obliquely inclined. Hinge without teeth. A conical 

 cardinal pit, situated below the beaks, partly external, opening 

 outwards and receiving the ligament. 



The plagiostoma differs from the pecten, by the beaks not 

 being contiguous, by the external and flattened facets of the car- 

 dinal base, and by the pit for the ligament opening, by a hole, 

 outwards. Except that it wants their two cardinal teeth, the 

 hinge of the plagiostoma resembles that of the spondylus. It 

 is distinguished from the lima by not gaping at either side, 

 whence it cannot be attached by a byssus ; for it is a mistake to 

 suppose that the external aperture of the ligamental pit serves 

 for the passage of that apparatus, a circumstance which never 

 occurs with the conchifera, and is incompatible with the dispo- 

 sition of the organs of the animal. The plagiostoma is only 

 known in the fossil state ; its shell is generally thin, even in 

 those of large size. This genus was first observed by Mr. 

 Sowerby. 



Type. Plagiostoma transversa t- 



Shell very large, transversely ovate, rounded above ; lower 

 sides oblique ; longitudinal furrows very numerous, transversely 

 striated. 6 Species. PI. II. Fig. 88. 



• Sijuamose. Laiiiairk's second Specie':,. His type is L. ivfiitta. 

 t From ll^ayivf, obWiw, cind ffrt/Ao., a month. t Transverse. 



