304 Annals of the Carnegie Museum. 



the edge is here sharp. The water-tubes (ovisacs) have a peculiar 

 curve backward toward a point near the posterior end of the mar- 

 supium. For the present, I cannot correlate this structure with any 

 special function, and must leave it for future study to decide what the 

 meaning of this feature is. 



We may arrange the genera of this subfamily as follows: 



ai. Marsupium thin, not kidney-shaped, ovisacs subcylindrical or very slightly 

 compressed. Placentae generally rather solid. Inner edge of mantle in 

 front of branchial opening not distinctly differentiated. Shell rounded, 

 ovate, or subelliptical, sometimes with outer sculpture. Male and female 

 shells practically alike. 

 61. Marsupium occupying the whole of the edge of the outer gill, folded. Pla- 

 centae subcylindrical, club-shaped, short, Shell subelliptical. 



Ptychobranchus. 

 b?. Marsupium occupying only a part of the outer gill. Placentae moderately 

 elongated or very long, subcylindrical, or very slightly compressed. 

 Shell more or less rounded, generally with tubercles. 

 a. Marsupium consisting of comparatively few, elongated ovisacs, ex- 

 tending from near the base of the gill beyond its edge. Placentae 

 subsolid, subcylindrical. 

 di. Placentae moderately long, slightly curved. Marsupium just behind 



the middle of the gill Obliquaria. 



di. Placentae very long, spirally coiled up. Marsupium in, or slightly 



in front of the middle of the gill Cyprogenia. 



d. Marsupium consisting of a large number of ovisacs, occupying the 

 larger posterior section of the outer gill. Placentae slightly com- 

 pressed near base, tapering, and becoming subcylindrical toward 

 the margin. 

 d\. Ovisacs straight. Placentae almost entirely beyond the edge of the 

 gill, subsolid. Glochidia much longer than high, arranged on the 

 outer faces of the marsupium. Edge of marsupium blunt, in old 

 specimens warped and folded. Shell rounded, disk nodular. 



Dromus. 



d-2. Ovisacs strongly curved back in the distal part. Placentae for a 



great part enclosed between the original gill lamina?, not solid. 



Glochidia not longer than high, distributed uniformly through the 



placental mass. Edge of marsupium sharp, pointed behind, not 



folded. Shell subelliptical, disk smooth Friersonia. 



as. Marsupium thick, more or less kidney-shaped, ovisacs dilated and compressed. 

 Placentae not very solid. Inner edge of mantle in front of branchial 

 opening, more or less differentiated. Shell rounded, elliptical, or elon- 

 gated, swollen or compressed, generally without any outer sculpture. 

 Male and female shells more or less different in shape. 

 61. Inner edge of mantle slightly lamellate and crenulated, but without dis- 

 tinct papillae or flaps. Male and female shell differing only slightly 

 in shape, sometimes hardly at all. 



