﻿ARCIDUNS '^75 



rays The beak sculpture is remarkably strong, often extend- 

 in' well on to the body of the shell in oblique ndges. The 

 po'sterior truncation between the two ridges may be square or 

 the upper point may project far beyond the lower one^ Ihe 

 teeth are much as in Alasmodonta, but the texture and decided 

 thickening of the nacre m front as well as the decided trun- 

 cation of all four of the gills behind are peculiar. 



Genus ARCIDENS Simpson, 1900. 

 Aradens Simpson, Syn., 1900. p. 66i.-Ortmann, Ann. Car. 

 Mus., VIII, 1912, p. 284. . . . , , , 



Shell subsolid, inflated, subrhomboid, with full, high beaks; 

 beak sculpture very strong, consisting of irregular corruga- 

 ^ons which fall into two loops, at the bases of which the ridges 

 are swollen into knobs that continue out m two radiating rovvs 

 on to the disk of the shell; in front of and behind the beaks 

 are many fine, radial wrinkles, the posterior ones being zig- 

 ^a-ed- surface of the shell covered with oblique folds and 

 wrinkle's; epidermis dark olive, shining; there are two elon- 

 crated, compressed pscudocardinals in the left valve, the pos- 

 terior under the beak, and curved upward, cutting oft the hinge 

 plate in the right valve, which has a single, compressed pseu- 

 docardinal in front; laterals nun.erous. short, blurred; muscle 

 scars irregular ; nacre white. Animal with the gi Is very large, 

 rounded below ; inner the larger, free from the abdominal sac ; 

 marsupium filling the outer gills, of a peculiar granular tex- 

 ture- palpi verv large, elongated, attached half way to the 

 mantle; mantle with a thick, unspotted border; branchial open- 

 ing large, papillose; anal opening with rudimentary papillae. 

 Type, Alasmodonta confmgosa Say. 



Arcidens confragosus (Say). 



Shell rhomboid, inflated, subsolid. or solid, slightly inequi- 

 lateral ; beaks high and full, their sculpture consisting of irreg- 

 ular, doubly-looped ridges, the bases of the loops being devel- 

 oped into strong, pinched-up nodules, which extend out on to 

 the disk of the shell ; in front of and behind the loops there are 



