BEAKS OF UNIONIDjE 175 



shells presented to the State Museum in 1886, by C. E. 

 Beecher, now in charge of the Department of Invertebrate 

 Palaeontology in Yale University Museum. The collection 

 is rich in immature stages of Unionidae, and in adult speci- 

 mens of the same family with beak characters well pre- 

 served. Of the eighteen specimens figured on the plate, 

 fifteen are from this collection. 



The Gould, Emmons, Aldrich, Lewis and Gebhard col- 

 lections have furnished much fine material for comparison, 

 all identified and labelled by the late Dr. James Lewis of 

 Mohawk, N. Y. The figured specimens of U. Tappanianus 

 and M. rzigosa are from the Aldrich and Gould collections 

 respectively. The specimen of An. subcylindracea shown 

 in fig. 13 is from the Lewis collection. 



Unio pressus, Lea 



Fig. 1 



Beaks with four or five prominent undulations, which are 

 acutely angular and high posteriorly; rounded and lower 

 anteriorly; each undulation having a deep sinus on the ven- 

 tral side back of the middle and plainly visible converging 

 lines both anteriorly and posteriorly. Dorsal areas with 

 several elevated, radiating lines, curved and wavering and 

 somewhat interrupted on the anterior area; direct and con- 

 tinuous on the posterior area. Anteriorly the converging 

 lines and radiating lines are much stronger than posteriorly. 



The beaks of this shell are usually more or less decorti- 

 cated, but in few cases only are the undulations themselves 

 eroded. The undulations are rather obcordate, with the 

 anterior wing larger and less angular than the posterior 

 wing. The sinus in the undulations of the beak is fre- 

 quently indicated throughout the growth of the shell from 

 the young to the adult, in the form of a slight linear depres- 

 sion traversing the shell from the beaks either to the middle 

 of the ventral margin or to a point immediately in front of 

 the posterior ventral angle. This species is nearly related 

 to Unio Tappanianus, Lea. 

 2 



