664 CRETACEOUS PALEONTOLOGY. 



1841. Hipponyx tentorium Mort, Jour. Acad. Nat. Sci. Phil., 



ist ser., vol. 8, p. 210. 

 1850. Helcion tentorium D'Orb., Prod, de Paleont., vol. 2, p. 



232. 

 1 86 1. Helcion tentorium Gabb, Synop. Moll. Cret. Form., p. 113 



(57)- 

 1864. Helcion ( ?) tentorium Meek, Check List Inv. Foss: N. 



A., Cret. and Jur., p. 17. 



1868. Halcyon ? tentorium, Con., Cook's Geol. N. J., p. 728. 

 1892. Helcion ? tentorium Whitf., Pal. N. J., vol. 2 (Monog. 



U. S. G. S., vol. 18), p. 153, pi. 19, figs. 6-8. 

 1905. Patella tentorium Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 19. 



Description. "Shell small, orbicular or subcircular in outline, 

 being slightly longer than wide, and measuring about half an 

 inch in length ; very depressed conical with a slightly anterior but 

 nearly subcentral apex which is elevated above the margin equal 

 to about one-third the length of the shell ; sides and anterior end 

 of the shell slightly concave between the apex and margin, and 

 slightly convex along the posterior side. Surface marked by 

 elevated, rounded, radiating costse, which are rather wider than 

 the interspaces and gradually increasing in size toward the margin 

 of the shell, but are constantly increased in number, both by 

 bifurcation and by implantation. The radii are crossed by very 

 fine concentric lines, but toward the margin of the shell. these 

 increase in strength so as to become distinct crenulations on the 

 top of the radii, and are nearly or quite one-half as strong as the 

 radii themselves." (Whitfield.) 



Remarks. This shell has much the aspect of one of the in- 

 articulate brachiopods, such as Discina, and according to Whit- 

 field the shell substance seems to be phosphatic, which would seem 

 further to suggest its brachiopod affinities. If it is one of the 

 gastropods, it belongs to the Patellidae, and would seem to belong 

 in the genus Patella, where it was originally placed by Morton, 

 rather than in the genus Helcion, where it has usually been placed 

 by later authors. The shell agrees with Patella in having a nearly 

 central axis, instead of a marginal one, as in Helcion. 



