GUELPH FAUNA IN THE STATE OF NEW YORK IO3 



named feature, but it appears that the fine longitudinal lines mentioned 

 above would point to the presence of longitudinal ridges in the neanic 

 stage, as they do in Spyroceras, in which Clarke 1 has shown from Spyro- 

 ceras bilineatum, that coarser longitudinal ridges, with advancing 

 growth and by interplantation, become changed into more numerous 

 uniform, fine, longitudinal striae. The presence of such fine, longitudinal 

 striae in Troch. desplainense allows us, hence, to infer the presence 

 of longitudinal ridges on the earliest parts of the conch. 



Trochoceras costatum Hall 



Plate 20, fig. 1, 2 



Trochoceras costatum Hall, Geol. Sur. Wisconsin. Rep't Prog, for 1860-1861 

 Trochoceras costatum Hall, N. Y. State Cab. Nat. Hist. 20th An. Rep't. 



1868. p. 360 

 Trochoceras costatum Hall, N. Y. State Cab. Nat. Hist. 20th An. Rep't. 



revised ed. 1870. p. 402, pi. 25, fig. 15 



With the specimens of Trochoceras desplainense from the 

 lower dolomite at Shelby, are several which are not only sinistral, but also 

 differ from the former in their loose coiling and closer arrangement of 

 costae, sufficiently to warrant their reference to T. costatum, another 

 species reported from the Racine limestone. 



Conch a low torticone, slightly asymmetric, but sufficiently to show 

 that it is sinistral, very closely coiled, with wide open umbilicus, expanding 

 moderately ; whorl beginning with circular section, becoming within the 

 first volution depressed convex and assuming an oval section ; position of 

 siphuncle not determined ; surface ornamented by strong annulations, 

 which cross the sides obliquely backward, in the earlier part of the whorl, 

 being slightly curved, with the convexity directed forward, and becoming 

 slightly sigmoidal on the later portion. On the venter they are curved 

 backward. The costae number nine within the first 10 mm of the whorl, 

 and the interspaces increase regularly till there are at the end of the 

 whorl five costae within 10 mm. 



1 Geol. Minnesota, v. 3, pt 2, p. 786. 



