UPPER TRIASSIC SPECIES. 127 



converging, and slightly convex in outline, to the acutely angular peri])hery, 

 on each side of which there is a very shallow undefined concavity 

 that can hardly be called a channel, while on the inner side they arc 

 abruptly truncated or inflected, and gathered into little subnodose wrinkles 

 at the umbilicus ; each turn enveloping about four-fifths of the next 

 one within. Aperture, as determined by transverse sections of the volu- 

 tions, compressed-subhastate, being acutely angular at the outer end, and 

 l^rofoundly notched on the inner side, for the reception of the next turn 

 within. Surface ornamented, in young shells of one and a half inches in 

 diameter, by small regular costaj, that bifurcate at or near the little promi- 

 nences or wrinkles at the margin of the umbilicus, after which they cross 

 the sides and curve very strongly forward as they approach the periphery, 

 where they become merely obsolescent lines, that are continued some dis- 

 tance forward almost parallel to the carina ; thus indicating the probable 

 presence of a narrow prolongation of the outer side of the lip at the aper- 

 ture. A few very small, pimple-like nodes are also scattered over the inner 

 half of the volutions at this stage of the shell's growth, while, as it increased 

 in size, the costse become less strongly defined and the little nodes more 

 numerous ; but farther around toward the aperture both nodes and costaj 

 gradually fade away, until it is probable that in large shells, a part, or possi- 

 bly the whole, of the surface becomes nearly or quite smooth. (Septa 

 unknown.) 



Greatest diameter of a specimen incomplete at the aperture, 1.40 inches; 

 convexity, about 0.G5 inch. 



This shell has an unusually acute, unserrated, peripheral keel, which, 

 so far as the specimen shows, seems to retain its sharpness both in the young 

 and in the adult. The concavity on each side of this keel is very shallow, and 

 merely so directed as to contribute to the thinness of the knife-like carinn, 

 rather than to impart any tendency to divide off another keel or even obtuse 

 ridge on either side. The little pimple-like prominences on the sides of the 

 volutions are almost entirely on the inner half, mainly on the little costte, 

 and are irregularly scattered, so as to show little or no tendency to arrange 

 themselves in spiral rows. The obscure wrinkles or little prominences 

 around the small umbilicus give it a somewhat puckered appeorance. 



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