510 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxii. 



gills only, forming a pad-like marsupium. The genus is now found 

 throughout a large part of the Palearctic region and North America 

 generally, excepting the Pacific drainage. The Unios of what seems to 

 be the Gibhosus group are abundant in the Tertiary of eastern Europe. 

 Vnio davilai from Eoumania is almost exactly like IT. gibhosus, a recent 

 species of the Mississippi Valley, and there were species no doubt closely 

 allied in the Tertiary of the western United States. There were forms 

 in Eastern Europe which seemed to connect Unio and Fleurobema, some 

 of which were like P. clava and formed a connecting link with Quadrida. 

 There were Quadrulas showing relationship with our Q. trigona, rvbi- 

 ginosa, j)ustulosa., pyramidata, and other forms. Psilunio craviovensis 

 looks much like our Pleurohema (csopus and there are other striking 

 exami:)les of old forms resembling recent ones. 



Now, in Quadrula, ZTnio, Alasmidonta^ and some of the other allied 

 genera we not unfrequently see species or specimens which have the 

 ordinary concentric or doubly looped beak sculpture, and in addition to 

 this a number of line, regularly radiating ridges in front or behind, or 

 sometimes at each end of the ordinary sculpture, an atavistic develop- 

 ment, probably. In some groups this character is quite constant; in 

 others it is only occasionally seen. I have never noticed it in any of the 

 higher genera in which the male and female shells are different. 



Up to this point the male and female shells had been essentially alike, 

 and the ovisacks were not distinctly outlined, but formed smooth pads, 

 the marsupium occupying practically the whole gill. In the American 

 waters probably, a great change now began to take place in some of the 

 Uuionidfe. The ovisacs of many forms became distinctly marked out 

 by a deep sulcus; a constriction developed in some instances around 

 near the base of the marsupium, inside and out, so that the lower end 

 of the ovisacs resembled little bulbs, and these were often filled with 

 pigment of a different color from the rest of the ovisac. Ptyehobran- 

 elms, with its folded marsupium, may have developed from Unio; 

 Cyprogenia and Obliquaria, with their narrov," central marsupia, and 

 Bromus, in which it is hung all around the outer gills in short, distinct 

 ovisacs, would seem to have descended from Quadrnla by their shell 

 characters. In all of these the shells of males and females are essentially 

 alike. In Medionidns there is sometimes quite a well-marked swelling 

 at the central or post-basal part of the female shell which is absent in 

 the male, but sometimes it is impossible to separate the shells of the 

 two sexes. The marsupium consists of irregular ovisacs just behind 

 or almost at the center of the outer gills. In the strange Tritogonia the 

 female shell differs remarkably from that of the male, having developed 

 a broad, posterior wing, filled with a flap of the mantle, while that of 

 the male is swollen, and truncate behind. Of the marsupia of this 

 strange form I know nothing. Oboraria, with short, solid, full shells, 

 sometimes having a slight post-basal inflation in those of the female, 

 and riagiola, with triangular shells, often showing the distinction 

 between male and female, recall to some extent Quadrula in shell char- 

 acters, but have the marsupia iu the hinder part of the outer gills 

 alone. The ovisacs are distinct, and there is a sulcus below, and this 



