NO. 1205. SYNOPSIS OF THE NAIADES SIMPSON. 745 



Genus PLEUROBEMA (Rafinesque, 182O) Agassiz. 



(Type, Unio clava Lamarck. 1 ) 

 Pleurobema RAFINESQUE, Ann. Gen. Sci. Phys., Brux, 1820, p. 313. 



Shell solid, triangular to rhomboid, usually with a prominent umbonal 

 region; beaks at or near the anterior end of the shell, incurved and 

 pointed forward over a small but well developed lunule; beak sculpture 

 coarse, consisting of a few irregular, often broken ridges, which curve 

 upward posteriorly; posterior ridge present, but low and rounded; 

 epidermis showing the rest periods plainly, tawny to olive, often orna- 

 mented with rays which show a tendency to break into square spots; 

 hinge rather strong, the plate generally narrow; pseudocardinals tri- 

 angular, ragged; laterals reaching nearly or quite to the pseudocar- 

 dinals, double in both valves, in the right valve the inner being smaller; 

 muscle scars deep, the posterior rounded; cavity of the beaks shallow; 

 nacre silvery ; male and female shells essentially alike. 



Animal having the inner gills much the larger, rounded below, free 

 from the abdominal sac for a part or all of their length; marsupium 

 occupying the entire outer gills, the ovisacs in some cases seeming to 

 be arranged in pairs; animal generally yellowish to salmon red, some- 

 times more or less brown or blackish. 



(Group of Pleurobema clava.) 



Shell solid, triangular; beaks high, generally anterior; beak sculp- 

 ture consisting of three or four broken, coarse, irregular ridges; epider- 

 mis yellowish or tawny, marked with broken green rays which show a 

 tendency to form square spots; pseudocardiuals often somewhat length- 

 ened and more or less parallel with the laterals. 



Animal having the inner gills the larger except at the extreme pos- 

 terior end, free nearly or quite their whole length from the abdominal 

 sac; marsupium occupying the entire outer gills; branchial opening 

 rather large, with small papilla ; anal opening with minute papillae or 

 crenulations. Animal dirty whitish to salmon. 



t PLEUROBEMA CLAVA Lamarck. 



*Unio clava LAMARCK, An. sans Vert., VI, 1819, p. 74. *DESIIAYES, An. sans 

 Vert., 2ded., VI, 1835, p. 537 ; 3ded., II, 1839, p. 669.* CONRAD, Monog., 1,1835, 

 p. 5, pi. in, fig. 1. *FERUSSAC, Gner. Mag., 1835, p. 28. * CONRAD, Pr. Ac. 



1 1 regret that I have not been able to examine more of the animals of this group, 

 especially those of the gravid females. The shells on the one hand approach very 

 close to those of Quadrula in appearance, and to Unio on the other hand. But all of 

 them have shallow beak cavities, while those of Quadrula are deep, and the embryos 

 of Pleurobema are contained in the outer gills only. The shells are generally more 

 solid and more triangular than those of Unio, and the pattern of coloring is different 

 from that of either genus. It stands between the two genera. I place Unio cesopus 

 and varicosus in Pleurobema with some hesitation, though in a specimen of the 

 former containing comparatively few embryos there were none in the inner gills, 

 and their beak cavities are shallow. 



