72 INVERTEBRATA OF MASSACHUSETTS. 



Hinge with only the vestige of a cardinal tooth, and no lateral 

 ones. Ligament long and rather large, j^artly concealed. Inte- 

 rior glossy white, wnth minute radiating lines. Length -/-g- inch, 

 height somewhat more, breadth y^ inch. 



It inhabits deep water, and is very frequently taken from cod- 

 fish, caught in Massachusetts Bay. 



There can be no doubt that this is identical with the British shell, 

 though the specimens I have seen are much smaller than the foreign 

 specimens usually are. 



I have arranged it under the genus Luci^na, as most of its charac- 

 ters pertain to it, and none of them seem absolutely to forbid. The 

 genus Cry'ptodon has, however, been formed by Turton to embrace 

 this shell, on account of its single tooth. 



Family CONCHACEA, Lam. 



Three cardinal teeth, on one valve at least ; the other with as many or 

 feiver ; sometimes ivilh lateral teeth. 



Genus CYCLAS, Lin. 



Shell small.) thin, ovate-globose ; hinge xoith two minute cardinal 

 teeth in each valve, but sometimes in one only ; also compressed, 

 lateral teeth ; inhabits fresh water. 



Cyclas si'milis. 



Shell oval, truncated at the extremities in young, and rounded in 

 adult specimens, convex, sub-equilatcral ; beaks slightly elevated ; 

 surface with conspicuous, conce7itric wrinkles ; epidermis dark 

 chestnut-brown. 



Figure 53. 



State Coll., No. 204. Soc. Cab., No. 1808. 



Cyclas similis, Say ; JVicholson's Encyc, Amcr. ed., iv. pi. 1, f. 0. 

 Cyclas sulcata, Lam.; ^ra. sans Vtrt., vi. 271. 



Shell sub-oval, nearly equilateral, varying much in its outline. 

 In the adult shell the extremities are broadly and nearly equally 

 rounded, the posterior side being somewhat the longest and most 

 pointed, the base very little curved ; valves very convex, re- 



