BEAKS OF UNIONID^E 1 87 



Anodonta subcylindracea, Lea 



Fig- 13 



Beaks with three or four very fine undulations, which are 

 rounded posteriorly and are placed very obliquely with ref- 

 erence to the hinge-line. Posteriorly, the undulations have 

 fine, direct converging lines, and the posterior dorsal area 

 has several fine radiating lines ; anteriorly, the undulations 

 terminate abruptly, and the anterior dorsal area lacks radi- 

 ating lines. 



Held with the anterior margin toward the observer and 

 slightly elevated, with the beak pointing over the shoulder, 

 the resemblance of the undulations to the point and first 

 three ascending threads of an ordinary screw is very strik- 

 ing. To simply say that the undulations of this species are 

 spirally arranged would be sufficient to distinguish it from 

 all other Anodons found in this vicinity. 



Anodonta implicata. Say 



Fig. 16 



Beaks with five or six undulations, which are rounded on 

 the summits and placed parallel to the hinge-line. Each of 

 the first three or four undulations has a gentle sinus on the 

 ventral side back of the middle. The last two undulations 

 lack the sinus. Posteriorly, each undulation has a very 

 prominent converging line ; anteriorly, converging lines are 

 lacking, but the undulations themselves continue up the 

 anterior area nearly to the hinge-line. Dorsal areas with- 

 out radiating lines. 



In adult characters ^4 ?z. implicata resembles An. fluvia- 

 tilis so closely that one would naturally expect to find a 

 very close resemblance between their umbonal characters. 

 Such is not the case however. A very deep sinus in each 

 undulation, of which the angle is very acute and in the 

 direction of the tip of the beak, is the most prominent beak 

 character of An. fluviatilis, and one which holds the atten- 



