66 THE NAUTILUS. 



does not form distinct placentae. Glochidia very large, of a 

 specific, peculiar shape. They have the general Anodontine 

 character, and possess the typical hooks, but the anterior and 

 posterior margins are strongly convex, so that the hinge-line 

 is considerably shorter than the length of the glochidium. 

 The general shape thus becomes almost transversely ellipti- 

 cal, with the upper margin straight in the middle, the lower 

 with a slightly projecting point, which bears the hook. 

 Length 0.40, height 0.36 mm. 



Color of soft parts whitish, mantle margin with square 

 black spots posteriorly, entirely black on the inside of supra- 

 anal and anal. 



The structure of the soft parts of this species is truly 

 Anodontine, and does not show any essential differences from 

 that of the genera Symphynota, Anodonta, Anodont aides, and 

 Alasmidonta. Thus we are to rely only on the shell charac- 

 ters. The most important one, the beak sculpture, clearly 

 places this species with the genus Alasmidonta, but the gen- 

 eral shape of the shell, chiefly the peculiar truncation at 

 the posterior end and the rather strong sexual dimorphism, 

 give it a rather isolated position. Simpson created the 

 genus Pegias for it, relying, as it appears, chiefly on the 

 shape of the shell. But shape of shell is rather variable in 

 the genus Alasmidonta, and I think enough justice is done 

 to this, if we regard Pegias as a subgenus of Alasmidonta. 

 The shape of the glochidia is unique, and although of the 

 common Anodontine type, the triangular outline is changed, 

 in consequence of the great convexity of the anterior and 

 posterior margins, into a transversely elliptical. But since 

 the glochidia also of other species of Alasmidonta show dif- 

 ferences in shape, also this character is hardly of more than 

 subgeneric value. 



In Simpson's diagnosis of the genus Pegias, the radial de- 

 pression in front of the posterior ridge is unduly emphasized : 

 it is very faint, and indicated only in the female. The state 

 ment that anal and supraanal are not separated is not correct. 



