INVERTEBRATE PALEONTOLOGY. 1>«)| 



In its general appearance, this species resembles somewhat A. borealis, 



lint it is always much smaller, more depressed, and destitute of thT' distinct 

 mesial carina, and six impressed, radiating lines, while the apex is farther 

 removed from the anterior side. It has been taken as the type of the genus. 



The specimen represented by our figure 5 e, of plate 18, differs slightly 

 in outline from the typical form, being more elliptic, and possibly may belong 

 to another species, though 1 have preferred to view it provisionally as a 

 variety of that here under consideration. 



Locality and position — Yellowstone River, 150 miles above its mouth; 

 in a bed containing a blending of the fossils of the upper part of the Fort 

 Pierre group, and the lower beds of the Fox Hills group of the Upper 

 Missouri Cretaceous series. 



Aiiisomyoii subovatus, M & H 



Plato 18, figs. 5, d, and 6. 



Helcion subovatus, Meek and Hayden ( l.-Tilj), Proceed. Acad. Nat. Sci. Philad.. V11J, lis. 

 Anisomijon subovatus, Meek aud Hayden (lH(ili), Am. Jour. Sci., XXVIII (2d ser.), 35. 



Shell elliptic-subovate, patelliform, rather thin ; extremities obtusely, 

 and nearly equally rounded; lateral margins nearly straight aud subparallel ; 

 summit rather depressed, located about half-way between the center and the 

 front margin ; slopes of the sides convex, and forming an angle at the summit 

 of about 80°; posterior slope more»convex than the anterior; surface marked 

 by faint, irregular, concentric wrinkles, and fine, obscure, subimbricating lines 

 of growth. 



Length, 1.1G inches; breadth, 0.81 inch; height. 0.39 inch. 



When viewed in certain lights, there may be seen near the posterior 

 margin of the only specimen of this species we have met with, extremely 

 faint indications of small, regular, radiating costse. This character, however, 

 is so very obscurely marked that it might be easily overlooked, and is perhaps 

 generally entirely absent, especially where the surface is a little worn or 

 weathered. 



The curve of the lines of growth show that, when young, this shell 

 must have been very nearly like the following species; the apex in the first 

 stages of its growth being more nearly central than in the adult. As it 

 advanced in age, owing to the more rapid growth of the posterior margin, 

 the summit was left in front of the middle. 



