GENERA OF SHELLS. 15 



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

 obhqiie, 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- 

 ciata, turgida, plicataria, rufescens, maculata, subplicata, 

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

 lanea, rufa, squalida, Brasiliana, donacia, depressa, lilacea, 

 trigonella, deltoides, crassatella, — similis, lateralis, oblonga, 

 solidissima, arctata, teilinoides, 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 litde 

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

 become very thick by age. 



C. kingicola, donaeina, 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, shows that idea to be erroneous. 



E. cardioides — striata. 



