1 84 BULLETIN OF THE NEW YORK STATE MUSEUM 



the posterior dorsal area, which is plicate in M. marginata 

 and smooth in M. fabula, and should perhaps be divided 

 according to this difference, yet the relation of the undula- 

 tion of the beaks is of much more importance than the mere 

 slight difference of outline. The grouping should aim to 

 show the relation of the beaks. 



The differences between the beaks of M. marginata and 

 M. fabula are so slight that it is difficult to point them out. 

 The only absolute difference between the two is in the 

 length of the lateral portions of the undulations and modi- 

 fications dependent upon this difference. In M. fabula the 

 lateral portion of each undulation is somewhat (about one- 

 third) shorter than the lateral portion of the corresponding 

 undulation of M. marginata ; the sinus in the second undu- 

 lation is not so prominent, and the posterior lateral extremity 

 of each undulation lacks the long and gentle slope which is 

 so striking in M. marginata. 



Margaritana rugosa, Barnes 



Fig. 10 



Beaks much less strongly undulated than in M. marginata 

 or M. undulata. There are three or four undulations, the 

 first three of which posteriorly, are acutely angular and 

 elevated ; anteriorly, they are less acutely angular and 

 lower, and each undulation, except the last, has plainly 

 visible, though very weak, converging lines, anteriorly and 

 posteriorly. Dorsal areas without radiating lines. First 

 and second undulations each with a sinus ; third, either 

 lacking the sinus or showing only slight indications of it ; 

 fourth undulation, when present, nearly obsolete, lacking 

 sinus and converging lines. 



The young shell shows no indication of the corrugations 

 on the posterior dorsal area which are characteristic of the 

 species in the adult state. A marked relationship exists 

 between the beaks of this species and those of M. marginata, 

 and this relationship is further strengthened by the corru- 



