BEAKS OF UNIONID^ 1 89 



Lewisii are noticed. It is believed that several of the 

 species of the foregoing group should be referred to An. 

 fluviatilis. 



Anodonta Lewisii, Lea 



Fig. 18 



Beaks with four or five undulations which are acutely 

 angular and high posteriorly ; rounded and lower anteriorly. 

 First two undulations continuous ; later undulations with a 

 moderate sinus back of the middle and nearly interrupted 

 at that point. Anteriorly, there are no converging lines, 

 but the undulations continue up the anterior area a short 

 distance ; posteriorly, converging lines are prominent. 

 Dorsal areas without radiating lines. 



In form, color and other usually described specific char- 

 acters, this species is sometimes absolutely indistinguishable 

 from An. subcylindracea, but the very great differences in 

 the characters of the beaks of the two species afford easy 

 means of separating them. 



Fig. 14 of the plate represents an adolescent specimen of 

 An. subcylindracea which cannot be distinguished from An. 

 Lewisii by any character but the ornamentation of the beaks. 



Fig. 15 is taken from a rather young specimen of An. 

 Lewisii for comparison with the above. 



A comparison of the beak of fig. 14 with the figures of 

 the beak of An. subcylindracea and An. Lewisii will show at 

 once that the specimen belongs to the former species. 



On the other hand, the beaks of An. Lewisii bear some 

 resemblance to those of An. fluviatilis, but there are differ- 

 ences which serve to distinguish the two species. In An. 

 Lewisii the undulations of the beaks are fewer in number, 

 more widely separated and much stronger than in An. flu- 

 viatilis. In An. Lewisii the later undulations become so 

 weak at the sinus that the part posterior to this point has 

 the appearance of a granule. In An. fluviatilis the undu- 

 lations at the sinus do not materially differ from the portions 

 immediately anterior and posterior to the sinus. 



