188 BULLETIN OF THE NEW YORK STATE MUSEUM 



tion at first glance. None of the undulations of An. impli- 

 cata have a deep sinus. The sinus, in the earlier undula- 

 tions, is very gentle, and in the later undulations it is absent. 

 Cases in which there is a striking similarity in adult 

 features between two species, such as that between An. 

 implicata and An. fluviatilis sometimes afford apt illustra- 

 tions of the importance of the undulations in making spe- 

 cific determinations. Typical specimens of the two species 

 are easily distinguishable by their form. All specimens 

 are not typical however, and a trifling variation in the char- 

 acters of An. fluviatilis or An. implicata, added to loss of 

 umbonal features by erosion or otherwise, is sometimes 

 sufficient to make each practically indistinguishable from 

 the other. When the undulations of the umbones are 

 present there can be no room for doubt in making an iden- 

 tification. 



Anodonta fluviatilis, Dillw. 



Fig. 17 



Beaks with five to seven fine undulations, which are 

 acutely angular posteriorly and gently rounded anteriorly. 

 Each undulation with a prominent sinus on the ventral side 

 about the middle, of which the angle is acute and in the 

 direction of the beak. Posteriorly, the undulations have 

 plainly visible converging lines. Anteriorly, no converging 

 lines are present, but the undulations continue up the 

 anterior area nearly to the hinge-line. Posterior dorsal area 

 with faint indications of one or two radiating lines. In the 

 lateral portions the undulations are scarcely more elevated 

 at one point than at another. 



This species and the following, An. Lewisii, are related to 

 a large group of Anodons, among which may be mentioned 

 An. plana, decora, ovata, Benedictii, Footiana, etc. As it 

 would add unduly to the length of this paper, no attempt is 

 made to point out the differences between An. fluviatilis 

 and its near relatives ; but under the next species the dis- 

 tinctions between the beaks of An. fluviatilis and An. 



