378 PKOCEEDINGS OF THE ACADEMY OF [May, 



nucleus. There is no indication of a periostracum, and the carti- 

 laginous shell is nearly transparent. 



The specimen was collected on the beach of St. Paul Island, 

 Bering Sea, after a severe storm, December 5, 1914, by Mr. G. Dallas 

 Hanna of the Bureau of Fisheries, in whose honor it is named. 

 U. S. Nat. Mus. Cat. No. 215,162. 



This species differs from 0. corys by its widely foraminate notseum, 

 its much larger visceral hump, its single instead of double osphradial 

 lamella, and the character of its shell. 0. gronlandica, glacialis 

 and pacifica have an imperforate notseum. 0. gronlandica has a low 

 visceral hump and entirely different form of verge. 0. glacialis has 

 a proportionately much shorter and posteriorly pointed foot, lower 

 hump and different type of verge, and 0. pacifica shows much the 

 same differences. 



The disposition to "lump" together specifically animals of this 

 genus in spite of minor differences, is responsible for much confusion. 

 The differences of more than specific rank run parallel to those in 

 Velutina, where some species have the shell entirely covered by the 

 notseum and others have it more or less exposed. It has been 

 suggested that all species have the shell covered in the young, but 

 the specimens of all ages collected by me do not confirm this sup- 

 position. 



While the edges of the shell and its concave surface, except for 

 the portion near the nucleus, are free from the visceral hump below, 

 it is nevertheless completely covered by an extremely thin layer of 

 tough tissue which can be separated from the shell only with difficulty, 

 usually coming off in small strips. This tissue I assume to be an 

 extension of the mantle as it has none of the characteristics of a 

 periostracum. The entrance of the vagina could not be made out. 

 It required for its demonstration more extensive dissection than was 

 thought advisable for the unique specimen. The animal is pre- 

 sumably hermaphrodite, like the other species of the genus which 

 have been anatomically examined. 



