12 18 PRISODON 



Subgenus Prisodon s. s. 



Characters as in the genus. 



Type, Mya synnatophora JMeuschen. 



Prisodon svrmatophorus (Meuschen). 



Shell subtriangular or subtrapezoid. more or less inflated, 

 solid, somewhat inequilateral ; beaks full, more or less elevated, 

 apparently without sculpture ; posterior ridge high, generally 

 narrowly rounded, often cord-like, occasionally semi-double 

 below, ending at the shell's base in a point or a narrow biangu- 

 lation ; dorsal line usually straight, ending in a wing before and 

 behind ; anterior end narrowed, cut away and rounded below ; 

 base straight or curved ; posterior end obliquely or almost 

 squarely truncate, its outline often incurved; surface having 

 delicate strise of growth and fine, radiating lines and sometimes 

 a few folds below the ligament; epidermis shining, greenish, 

 greenish-yellow or brownish, often with metallic tints ; teeth 

 elongated, rather solid but compressed ; pseudocardinals two or 

 more in each valve ; one lateral in the right valve and two in 

 the left, all of them sometimes slightly vertically striate ; muscle 

 scars not deep ; nacre bluish, flesh-colored or purplish. 



Length 102, height 70, diam. 35 mm. 



Length 85, height 52, diam. 34 mm. 



Guiana ; Brazil. 

 Triquetra siibtirdis Ki.ein, Meth. Ost., 1753, p. i35; pi. ix, 



fig. 36.— H. and A. Adams, Gen. Rec. Moll., IL 1857, p. 505; 



III, pi. cxx, figs. I, 10. 

 Margaron (Triquetra) snhiiridis Lra, Syn., 1852, p. 17; 1870, 



P- 25. 

 Mya syrmatophora 1\Ikitschk;n in Gronovius, Zooph., 1781, 



pi. xviii, fig. I, 2. — Wood, Ind. Test., 1825, p. 12, pi. 11, 



fig. s6b. 

 Hyria synnatophora Sowkrhy, Rec. and Foss. Shells, XVL 



1823, fig. d; Conch. Man., 1839, fig. 143. — ReKVE, Conch. 



Syst., T, 1841, p. 120. pi. xc, fig. I. — Wood. Ind. Test, rev., 



1856, p. 17. pi. II, fig. 36. — HuPE. Moll. Nouv., Ill, 1857. 



p. 79, pi. XV, fig. 2. — SowERBY, Conch. Icon., X\^IT, 1869, 



pi. v, fig. 2. 



