288 IIN1TKD STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



families, but would apparently occupy a place in the widely-removed Fis- 

 surellidce. Of this, however, there is, I now think, no probability. 



This genus seems to range back to the Jurassic, and certainly to the 

 Cretaceous epoch, though until the rather numerous fossil shells that have 

 been described by palaeontologists under the names Patella, Acmcca, Helcion, 

 &c, have been more carefully studied by the aid of specimens giving a cor- 

 rect knowledge of the interior, and the nature of the apex, it will not be 

 possible to determine its exact geological range, or to give a list of the species. 



I would suggest a thorough and critical examination of internal casts, 

 to any person who may be disposed to undertake the task of eliminating the 

 species from the several genera to which doubtless a number of the Creta- 

 ceous, and perhaps some Jurassic (or possibly even older species), have been 

 referred. It is very rarely, indeed, that the interior of such thin fossil shells 

 will ever be clearly seen ; but well-preserved internal casts nearly always 

 show impressions of the muscular scars. 



I have the impression that this genus did not survive the close of the 

 Cretaceous epoch 



Anisoinyon b o r e a 1 i s , M. & H. 



Plate 18, figs. 9, a, b, c, d, e. 



Hipponyx borealis, Morton (1842), Jour. Acad. Nat. Sci. Pbilad., VIII, 210, pi. xi, fig. 6. 

 Helcion carinatus, Meek and Havden (1856), Proceed. Acad. Nat. Sci. Pbilad., VI, 68. 

 Aiiisomyon borealis, Meek and Haydeu (1860), Am. Jour. Sci. aud Arts, XXVIII (2d ser.), 35. 



Shell thin, bonnet-shaped, the summit being prominent, with a general 

 forward obliquity, and located in advance of the middle; immediate apex 

 very small, abruptly pointed, and having the characteristic abrupt backward 

 curve well-marked ; base nearly circular anterior and anterior-lateral slopes 

 abrupt and distinctly concave ; posterior and posterior-lateral slopes convex, 

 the former being carinated along the middle, and the latter usually supporting 

 a few very obscure radiating ridges; surface, when slightly worn, appearing 

 smooth, but in well-preserved examples marked by fine, inconspicuous lines 

 of growth, which are crossed by similar closely-arranged, radiating stria-, 

 scarcely visible without the aid of a magnifier; radiating from the apex may 

 also be seen six equidistant, deeply-impressed hair-lines, four of which pass 

 down the posterior and lateral slopes, and two down the anterior-lateral. 



Diameter of aperture or base, 1.93 inches; height of apex, 0.80 inch. 



