1913.] NATURAL SCIENCES OF PHILADELPHIA. 665 



posterior wing being distinctly twisted to the left side; posterior 

 ridge not prominent, indistinctly duplicated, producing an indistinct 

 biangulation; posterior groove scarcely perceptible; superior margin 

 slightly arcuate, anterior narrowed, sharply curved above and 

 gradually and regularly extending into and through the basal margin 

 in an evenly decreasing curve to the lower, posterior angle; posterior 

 margin obliquely truncate; hinge line sharply curved; right valve 

 with three pseudocardinals, two small, simple, widely diverging, 

 including between them a large, triangular middle pseudocardinal 

 which is split into six irregular denticles, and a long, narrow, low- 

 arched lateral lying well within the margin of the posterior wing; 

 left valve with three pseudocardinals, the anterior simple and promi- 

 nent, the posterior two flattened and irregularly divided into denti- 

 cles, and two long laterals which coalesce anteriorly over about one- 

 third of their length; anterior muscle scar rather evenly rounded, 

 well impressed, posterior elongate and indistinct; pallial line well 

 marked; nacre pearly-white, iridescent posteriorly; ligament mod- 

 erate, thickened posteriorly. 



Animal unknown. 



Length 45.75, width 31, diam. 11.75 mm. 



A young specimen of this species which seems to be new, taken 

 in the Rio Jamauchim, was sent from the Goeldi Museum. The 

 sculpture suggests H. rugossissima Sby., but is very much finer than 

 in that species. 



Genus PRISODON Schumacher. 

 Frisodon obliquus Schumacher. 



Ess. Nouv. Syst., 1817, p. 139, pi. xl, fig. 2. 

 Simpson, I.e., p. 871. 



One specimen taken at Boim, on the Rio Tapajoz, was sent from 

 the Goeldi Museum. 

 Prisodon syrmatophorus (Meuschen). 



Mya syrmatophora Meuschen, in Gronovius, Zooph., 1781, pi. xviii, figs. 1, 2. 



Prisondon (,yrmatophorus (Meuschen), Simpson, I.e., p. 870. 



Seventeen specimens taken in the Rio Jamauchim were sent from 

 the Goeldi Museum. 



Genua DIPLODON Spix. 

 Diplodon kelseyi n. sp. PI. XXVII, figs. 5, 6, 7. 



Shell transversely subrhomboidal, rather heavy, of moderate size, 



very inequipartite; epidermis dark brown to black, rayless and 



almost lustreless; dorsal margin markedly curved, the basal nearly' 



straight, narrowly, evenly rounded in front, obliquely truncate 



44 



