UPPER TRIASSrC SPECIES. 117 



in form, with the peripheral channel of somewhat variable depth, but usually 

 well-defined; volutions about five, more or less flattened on the sides, round- 

 ing off a little to the perif)hery, and abruptly truncated, or even overhang- 

 ing on the umbilical side, each enveloping about half of the next one 

 within; umbilicus rather shallow, or of moderate depth, and less than one- 

 third the diameter of the shell. Surface ornamented by a variable number 

 of costse, which curve forward as they approach the periphery, often bifur- 

 cating once or twice in crossing the sides, and also increasing by the inter- 

 calation of shorter ones between, while they usually bear on each side about 

 four or five rows of small nodes, the most prominent of which are generally 

 those along the margins of the peripheral furrow, and around the edge of 

 the umbilicus; the outer rows being slightly compressed, and more or less 

 oblique, while those on the sides of the volutions are apparently sometimes 

 obsolescent. (Septa not observed.) 



Greatest diameter of the largest specimen seen (which is imperfect), 

 about 4 inches; convexity, 1.20 inches; diameter of umbilicus, about 1.15 

 inches. 



The specimen from which the foregoing description and our figures 

 were prepared, agrees much better with Mr. Gabb's fig. 12 than with his 

 fig. 1 1 , He was probably right, however, in including both of his figured 

 specimens as varieties of one species, though I have not seen a series con- 

 necting the two forms. Taking the specimen represented by his fig. 12 as 

 the typical example of the species, our shell may safely be called T. Whitneyi. 



Among foreign species, this may be compared with T. Archelaus, Laube 

 (Fauna der Sch. von St. Cassian, 5. Abth., pi. xl, fig. 1); but it differs jn 

 having its cost£E more frequently bifurcating, with fewer nodes, none of 

 which seem to be developed into spines, as in liaube's species. 



After I had written the foregoing. Professor Hyatt sent me the follow- 

 ing note respecting it: 



" This species is very distinct* on account of its coarse prominent 

 nodes and pilse, the number of the latter, and the young, which are not 

 unlike the young of Gymnotoccras Blalcei. The pilse of the young shell in 



*He lueaus distinct from T. Judicaricum ; be concurred in the opinion that it is 

 the T. Whitneyi, Gabb (sp.). 



