﻿324 iii:.mii.asti;na 



!\Iarsu[)iuiii filling- the entire outer gills and forming- enor- 

 mously thickened pads, the upper part finely vertically striate, 

 the lower part of different texture, lighter colored, wrinkled 

 and granular on the surface: embryos very large; outer and 

 inner gills nearly alike in size, the latter free from the abdomi- 

 nal sac. all imited to the mantle to their posterior ends ; palpi 

 elongated, granular: mantle straight below, thickened on the 

 border ; branchial opening large, with numerous light-colored 

 papilla? with dark lines : anal opening black, without papillae, 

 separated from the small superanal opening by a long bridge; 

 hinder part of the mantle, branchial and anal openings widely 

 separated by the very thick marsupia. 



Type, Alasmodonta ainbigua Say. 



From the examination of a large series of fine specimens 

 sent by Mr. L. E. Daniels, I can only conclude that this curious 

 form is quite different from any other I know. The marsu- 

 pium is decidedly vertically striate throughout its upper half, 

 the lower half is lighter, granular and wrinkled externally 

 and in places shows traces of vertical sulci as in the higher 

 Uniones, Lainpsilis, etc. The males seem to be much less 

 numerous than the females, only two being found in the lot 

 sent by Mr. Daniels. Their shells are less inflated, more nearly 

 straight below and not so broad posteriorly ; those of the fe- 

 males being decidedly swollen behind, the posterior ridge being 

 very full and widely rounded, the base line sometimes a little 

 incurved in the middle and the hinder end of the shell broad. 



The shells of this little species bear a strong superficial 

 resemblance to those' of the genus Margaritaiia. They difi^er, 

 however, in several minor characters and one of the most 

 important is in the beak sculpture. That of Hcuiilastcua has 

 tine ridges, which are looped up in the middle and pass down- 

 ward and backward obliquely and are open behind, while the 

 sculpture of Margaritana consists of strong, straight folds. 

 Tn the latter genus the mantle is more or less attached to the 

 cavities of the shell by small muscles, which leave scars when 

 the animal is tciken out, a character not found in Ifemilastcna. 



