BEAKS OF UNIONID^E 1 83 



more elevated posteriorly, in showing less distinct indica- 

 tions of a sinus in several of the undulations and in having 

 the summits of the undulations sharp instead of rounded. 

 Under the description of U. ochraceus the points of 

 resemblance and difference between U. cariosus and U. 

 ochraceus are pointed out. 



Margaritana marginata, Say 



Fig, 9 



Beaks with three or four very coarse and rude undula 

 tions, which are placed nearly parallel to the hinge-line. 

 Each undulation with a gently curving sinus on the ventral 

 side and very fine converging lines both anteriorly and pos- 

 teriorly. The converging lines of the first two undulations 

 are plainly visible ; those of remaining undulations are 

 nearly obsolete. The posterior lateral portion of the last 

 undulation is usually obsolete. Dorsal areas without radi- 

 ating lines. 



Perfect specimens of this species may be easily secured. 

 As a rule the Margaritanae of this vicinity suffer much less 

 erosion than the Unios or Anodons, and this statement 

 appears to hold good for other localities. 



In many specimens of this species the sinus in the undu- 

 lations of the young shell is indicated in the adult by a 

 depression of the surface of the shell from the beak to the 

 middle of the ventral margin and by a slight bend in the 

 margin. The posterior dorsal area of the young shell some- 

 times shows the corrugations which are characteristic in the 

 adult. 



This species is closely related in beak characters to M. 

 fabula. Dr. Lea, in his disposition of these two species, 

 gives another example of the artificial character of the last 

 division of his classification, viz.: the grouping according to 

 shape of outline. M. marginata is placed under plicate, 

 oblong Margaritanae, and M. fabula under smooth, arcuate. 

 Although the two species differ materially in the nature of 

 3 



