48 GENERA OF SHELLS. 



ostrea by being regular, free, and almost equivalve, and 

 from pecten by its remote beaks and cardinal pit. They 

 are sea shells, and generally white. 



L. inflata, squamosa, glacialis, annulata, fragilis, lin- 

 guatula. 



PLAGIOSTOMA. Shell subequivalve, free, sub- 

 auriculated, cardinal base transverse, straight; 

 beaks rather distant, their inner sides expand- 

 ing into transverse, flattened, external facets, 

 one straight, the other obliquely inclined. 

 Hinge without teeth ; a conical, cardinal pit, 

 under the beaks, partly internal, opening out- 

 wards, and receiving the ligament. 



It is only known in the fossil state. 



PECTEN. Shell free, regular, inequivalve, 

 eared ; inferior edge transverse, straight ; beaks 

 contiguous. Hinge toothless ; cardinal fossa 

 wholly interior, triangular, receiving the liga- 

 ment. 



The pectines are almost always radiated with fine or 

 coarse ribs. The base of the shell is terminated by a 

 straight, transverse line, beyond which the beaks never 

 project. The valves are generally thin, of equal size, 

 but the upper one almost constantly flattened. They 

 are always eared, and the largest ear is on the posterior 

 side, and beneath it is a sinus. 



(a.) Ears equal, or nearly so. 



P. maximus, medius, Jacobaeus, bifrons, ziczac, Laurentii, 

 pleuronectes ? obliterates, Japonicus, magellanicus, purpura- 

 tus, lineolaris, radula, rastellum, turgidus, flagellatus, asper- 



