GENERA OF SHE'ULS.. 15 



If the pit be very large, the cardinal tooth is very 

 oblique, short, and often partly obliterated ; but the 

 lateral teeth always exist. 



M. gigantea, spengleri, striatella, carinata, helvacea, gran- 

 dis, stultorum, maculosa, straminea, australis, violacea, fas- 

 eiata, turgida, plicataria, rufescens, maeulata, subplicata, 

 iriangularis, lactea, abbreviata, ovalina, alba, solida, cas- 

 tanea, rufa, squalida, Brasiliana, donacia^ depressa, lilacea, 

 trigonella, deltoides, crassatella, similis, laleralis, oblonga, 

 solidissima, arctata, tellinoides, nucleus. 



(2.) Shell not gaping at sides. 



CRASSATELLA. Shell inequilateral, subor- 

 bicular or transverse, closed ; cardinal teeth 

 two, subdivergent, with a lateral pit ; ligament 

 interior inserted into the pit of each valve ; 

 lateral teeth wanting or obsolete. 



Crassatella is distinguished from mactra and lutraria 

 by the valves, when shut, being quite close on both 

 sides. In a few species, the ligament appears a little 

 on the outside. They are all sea shells, and generally 

 become very thick by age. 



C. kingicola, donacina, sulcata, rostrata, glabrata, subra- 

 diata, contraria, cuneata, erycinaea, cycladea, striata. 



ERYCINA. Shell transverse, subinequilateral, 

 equivalve, rarely gaping ; cardinal teeth two, 

 unequal, diverging, having a pit interposed ; 

 lateral teeth two, oblong, compressed, short, 

 entering ; ligament interior, fixed to the pit. 



One of the cardinal teeth joining the base of the 

 lateral tooth, has been sometimes mistaken for a bifid 

 tooth ; but the corresponding hollow on the opposite 

 valve v shows that idea to be erroneous. 



E. cardioides striata. 



