52 THE NAUTILUS. 



Truncilla. This. would be a quite natural genus, eliminating some 

 lieterogenous elements, that must be placed with Quadrula, as T. 

 personata Say, allied to Q. trigona, and T. perplexa andfoliata allied 

 to Q. metanevra. 



Lampsilis. A very natural genus, from which, however, Proptera 

 should be separated as a genus. 



Obovaria belongs in the vicinity of Quadrula, as do likewise Tri- 

 togonia, Cyprogenia, Obliquaria and Dromus. 



Ptychobranchus is a subgenus of Lampsilis. 



Pleurobema will form a natural group after the removal of P. 

 cesopus and probably some other species. 



Unio. There are to be removed U. spheniopsis, a species allied to 

 Nephronaias cyrenoides, and also the group of U. semigranosus, con- 

 sisting of species of Quadrula. There are a number of species of 

 Quadrula in Mexico distinguished by peculiar granular sculpture 

 and thin reddish-gray epidermis, for which Crosse and Fischer cre- 

 ated the sub-genus Psoronaias. I can find no reason for dividing 

 them between two groups, as Simpson has done; and I unite U. 

 semigranosus and allied species to Quadrula. In this way all sculp- 

 tured forms are eliminated from the genus Unio. 



Plagiola. Genus probably to be restricted to P. elegans and 

 similar forms, while P. cyrenoides, etc., may be transferred to 

 Nephronaias. 



The family Unionidce, as limited by Simpson, would contain, 

 according to these views, two families : DiplodontidcE and Unionidce, 

 which are distinguished not only by the marsupia. but also by im- 

 portant conchological characters. The sexual differences, strongly 

 pronounced in the Lampsilis group, are not entirely deficient in the 

 other groups, as there are species with pronounced sexual differences 

 in both the genera Quadrula and Diplodon. The sculpture of the 

 beaks is concentric in the Unionidse, radial in the Diplodontidce. In 

 some species of ffyriaand Tetraplodon the radial sculpture is reduced 

 or almost obsolete. Of two specimens of Castaliella sulcata Krauss in 

 my collection, one shows the radial sculpture well developed ; in the 

 other it is nearly absent. The pseudo-cardinals are variant in both 

 examples ; the nacre is bluish-white. This mussel is doubtless identical 

 with Tetraplodon schombergianus Sow., but 1 have received my speci- 

 mens, collected by Kapler on the Marowini River, Surinam, from the 

 Zoological Museum in Stuttgart as typical specimens of Castalia 



