4o6 GEOLOGY OF THE BLACK HILLS. 



ANISOMYON BOEEALIS. 



Plate 12, figs. 21-23. 



Hipponyx borealis Mort., Jour. Acad. Nat. Sci. Phil., vol. 8, 1842, p. 210, PI. xi, fig. 6. 

 Helcion carinatus M. & H., Proc. Acad. Nat. Sci. Phil., vol. viii, ]). 68. 

 Anisomyon borealis M. & H., Am. Jour. Sci. and Arts, vol. 28, 2d ser., p. 35. 

 Anisomyon borealis (M. & H.) Meek, Pal. U. S. Geol. Surv. Terr., p. 288, PI. 18, fig. 9. 



Shell of large size, subcircular or very short ovate in outline, widest 

 behind the middle. Summit elevated ; apex pointed or slightly recurved, 

 laterally compressed, and in the specimens in hand almost overhanging the 

 anterior margin of the shell ; anterior slope short, abrupt, and concave ; 

 posterior slope long and convex, the greatest elevation of the shell being a 

 little behind the apex ; lateral slopes concave in front of the middle and 

 convex behind ; surface of the shell angulated along the middle of the 

 posterior dorsal region, and also between this and the sides, the median 

 angulation the most extreme. Also by indistinct, irregular, flexuous, radiat- 

 ing lines of varying strength, and by stronger concentric lines of growth. 

 There are also six distinct hair-like radiating grooves passing from the apex 

 toward the margin at subequal distances from each other, which appear to 

 be mostly superficial, although sometimes visible on partially exfoliated 

 specimens. Muscular impressions not observed. 



On one of the specimens referred to this species, and which is a partial 

 cast of the interior, there are three radiating ridges passing from the apex 

 to a considerable distance along the surface ; one of them being situated 

 near the median angulation, the others, rising from below and in front of 

 the apex, pass along the sides of the cast for more than half an inch. The 

 former of these ridges we judge to be only a line indicating a fracture of 

 the shell ; the others are evidently a part of the muscular markings, and 

 most probably represent the anterior connecting muscular band seen on 

 other species. Here, however, it crosses in front and just below the apex 

 of the shell. 



Judging from the figures given by Mr. Meek, the species is a very 

 variable one ; our specimens are of the form represented by his profile 

 figure 9 b, but less elevated. 



Formation and locality. — In limestone of the Fort Pierre' Group, Chey- 

 enne River, Black Hills. 



