44 THE NAUTILUS. 



A. cygnea (Linnaeus), except in being slightly smaller (in 

 cygnea they are about 0.35 to 0.36 mm. ; in A. grandis and 

 cataracta, the glochidia are still larger, 0.36 x 0.37 ) . 



This species is also in shell characters closely allied to the 

 European A. cygnea. The soft parts have been previously 

 described by Lea (Obs., 10, 1863, p. 454). 



ANODONTA MARGIN ATA Say. (See Simpson, 1900, p. 632). 



One gravid female, collected August 4, 1912, by 0. E. Jen- 

 nings in Six Mile Lake, Silver Islet, Thunder Cape, Canada 

 (North shore of Lake Superior). 



The soft parts agree in every respect with those of A. 

 grandis and cataracta. The specimen had only eggs and no 

 glochidia. 



Lea's description of the soft parts of A. fragilis (=mar- 

 ginata) differs in giving the posterior margins of the palpi 

 united nearly the whole length. In my specimen they are 

 united for about one-fourth on the right side, and for nearly 

 one-half on the left, agreeing also in this respect with A. 

 grandis. 



THE SUBGENERA OF ALASMIDONTA Say. 



Simpson 's division into subgenera does not seem, to be quite 

 satisfactory, some closely allied species being separated in it. 

 The subgenus Bullella is not known in its anatomy, but from 

 shell characters it is very near to the subgenus Alasmidonta. 

 The species A. hoist onia is, as has been shown above, a Sym- 

 phynota. The genus Pegias Simpson should fall as a subgenus 

 in this genus. It is further to be remarked, that the subgenera 

 Pressodonta, Alasmidonta, and Pegias are more closely related 

 to each other than to the two other subgenera. 



a 1 Lateral hinge-teeth present, but their number reversed, two 

 in right, one in left valve. Beak sculpture moderately 

 heavy, bars with an angle upon the posterior ridge, and 

 a slight sinus in front of it. Inner lamina of inner gills 

 free. Female shell recognizable by a slight swelling in 

 the region of the posterior ridge. 



Subgeuus : Prolasmidonta nov. subgen. 

 Type: A. heterodon (Lea). 



