332 The Ohio Naturalist. [Vol. XI, No. 6, 



It is known that the ova, from the ovary, pass through an 

 oviduct on each side into the branchiae, where they develop into 

 embryos, the so-called glochidia. The glochidium, of about the 

 size of the ovum, has a two-valved shell, very different from the 

 postembryonal shell, and also of markedly different formation in 

 the several groups, and a very primitive formation of the soft 

 parts, without alimentary canal, ganglia, branchiae, etc. 



The formation of the female reproductive branchiae is varied 

 and furnishes principal characters for classification. In some of 

 the groups, the Unioniiiae (Ihiio, Pleurohcma, Quadrula), also the 

 Anodontinac (Aiiodoiita, Alasmidonta, Gymphynota. etc.), the 

 branchiae which receive the ova, in their whole extent, show 

 only slight and macroscopically barely noticeable differences from 

 the male branchiae, and the non-receptive of the female. In a 

 still higher group, only a part of each of the outer branchiae is 

 noticeably differentiated, the so-called marsupium, consisting of 

 ovisacs, their number being very different in the several groups, 

 and approximately constant in adult individuals of each species. 

 Also their configuration shows differences, when barren, and much 

 more so when charged. This is the groui), or subfamily Lanip- 

 silinae, and, with some differences, Proptera. In Ptychohranchus 

 (e. g. phaseolus Hildreth), the outer branchiae are differentiated in 

 their whole extent, and of a formation markedly different from 

 that of the others, when gravid. 



In the lower forms, there are no or slightly marked differences 

 of the shells between males and females. With the appearance of 

 the marsupium which, when filled and distended, projects more 

 or less over the general contour and the lower edge of the branchiae, 

 there comes a corresponding distension of the shell in the female, 

 not or slightly marked in some forms, strongly so in others, e. g., 

 most of the species of Lampsilis. It reaches its highest grade in 

 TrunciUa, W'here that part of the female shell is not only greatly 

 distended but also of a fomiation and sculpture different from 

 the rest of the mussel. 



These differences, gradations, of both soft parts and shell, are 

 naturally not in a straight line, the same as in other groups of 

 animals, but with ramifications and gaps, which latter would 

 probably be bridged over by extinct forms, and possibly by such 

 as are living in other zoo-geographical provinces. 



In connection wath the different fomiation of the gravid 

 branchiae, there are also different w^ays of discharging the embryos. 

 In the Unioninae the young are expelled upward from the brood 

 chambers into the suprabranchial canal and from there out into 

 the water through the anal siphonal mantle opening. But in the 

 Lampsilinae, each ovisac opens, at its inferior end, and 

 the contents, coherent as a cake ("placenta"), makes its exit 

 through that rent, and out either through the branchial siphonal 



