THE AMERICAN NATURALIST. [Vol. XXXV I L 



atic position and phylogeny of all groups of animals, and must 

 be taken into account in studying the Unionidae. Since differ- 

 ences between embryos seem to be correlated with circumscribed 

 groups of the adult animals, they must be regarded as a valuable 

 factor in classification. The writer has examined embryos of 

 many species at different stages of development, not only in 

 regard to the shells but also to the soft parts showing decided 

 and interesting differences between the several groups and 

 species. 1 There are three main types of glochidia, so far as 



1. Those of Anodonta, Alasmidonta, etc. — Their valves are 

 large, rounded-triangular, with a long dorsal commissure, a strong 

 thickening along the margins, and are pointed in the middle of 

 the ventral side, each bearing a large, rough spine, or "hook" 

 (spur) bent inward so as to prevent the closing of the shell, 

 which remains wide open during the embryonal stage of life. 

 This formation is of decided importance, morphologically and 

 physiologically, tor doubtless these strong spines facilitate the 

 attachment of the young mussels, after being discharged from 

 the branchiae of the parent, to the fins, etc., of fish, and possibly 

 to parts of other aquatic animals. It is to be regretted that so 

 little is known in regard to the early postembryonal life of the 

 young Naiades. 



2. Those of Proptera, e. g, alata Say. — The dorsal commis- 

 sure is short, the dorse-ventral diameter being the largest; the 

 ventral margins are rather short, truncated or slightly curved ; 

 there is a smooth spine at the anterior and posterior ends (of the 

 ventral margins) of each valve, bent slightly outward and by 

 passing those of the opposite valves, allows the shell to close 

 along the ventral side. The long anterior and posterior sides are 

 truncate, nearly straight, and the shell is widely gaping at both 

 ends, a feature which is permanent, to a degree, in the adult 

 shell. 



3. The glochidia of those species which were generally ranged 

 under the old genus Unio, with the exception of Proptera, have 



