96 THE NAUTILUS. 



The true Unios, which in the United States are well represented 

 by such forms as U. complanatus, U. buckleyi, U. crassidens and U. 

 gibbosus, liave longer shells than the Quadrules, they are generally 

 less solid, and as far as I have seen, the beak cavities are compara- 

 tively shallow, while the hinge plate is never wide and flat as in 

 Unio pustulosus. In these the embryos are found in the outer gills 

 only. 



Mr. Call uses the time honored names Unio, Margaritana and 

 Anodonta for the Indiana Unionidaj. These names are applied in a 

 subgeneric sense, but he neglects to tell us what genus he places them 

 under, whether it is Unio of Retzius or the Margaron of Lea. In 

 his artificial key to the groups of Unto, excluding Anodonta and 

 Margaritana, he places the species in groups, typified by U. hiteohis, 

 U. ligamentinus, U. crassidens, U. tubercrdatus, U. personatus, etc. 

 These groups have been recognized as natural assemblages by Lea, 

 Lewis, Marsh, Wetherby, and most of the other American students 

 of the Unionida?. I consider them as natural and reasonable as any 

 that can be made among large assemblages of nearly related species, 

 and every amateur who gets together a collection of naiades begins 

 to perceive them after a little study. Yet Mr. Call seems to con- 

 sider them as mere artificial or laboratory devices of little value. 



The Anodonta edentula of Say is placed by Mr. Call in th^ genus 

 Anodonta. In this remarkable species there are more fully devel- 

 oped vestigeal hinge teeth than in any of the true anodontas, and the 

 hinge plate is incurved in front of the beaks, while in Anodonta 

 proper it is evenly curved throughout. This species carries the 

 young in the outer gills in short, distinct ovisacs running directly 

 across the gill, instead of in long ovisacs running vertically, as in the 

 true anodont-as. These contain from 8 or 10 to 20 or more embryos 

 and finally break out through the outer walls of the outer gills and 

 are discharged entire, with their contents, into the water. After this 

 the gills assume the ordinary unionoid form. It is probable that a 

 few other species from the southern states Avliich have been placed in 

 Margaritana have similar marsupia. I have examined gravid speci- 

 mens of most of our American anodontas and of the A. woodiana 

 Lea of China, and in all of them the marsupia are radically different 

 from that of A. edentula. 



Mr. Call's synonymy of Unio clavtis Lamarck is certainly aston- 

 ishing, and includes species as different in shell characters as can be 

 found among the North American unios. C. T. S. 



