Geol— Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 39 1 



ribs usually do not bifurcate. The knots bordering the venter do not lie 

 opposite each other, but in alternate position. 



At extreme maturity the ribs become weaker and the knots stronger, so 

 that young shells and mature shells are quite different in appearance. 



The ventral furrow is quite distinct, although the ribs may still be traced 

 across it. In adolescence and early maturity the furrow is not developed, 

 and the ribs on the venter are strong. In this character the species is inter- 

 mediate between Anolcites and Protrachyceras. 



Meek identified this species with Trachyceras jiidicariciim 

 Mojsisovics of the Alpine province, from which it differs 

 greatly in its coarser and fewer ribs and knots. 



Horizon and locality. Middle Trias, Daonella beds, 

 West Humboldt Range, Nevada, Cottonwood Canyon, and 

 on the divide between Troy Canyon and the south fork of 

 American Canyon. The figured specimens were collected 

 by the writer at the last locality, associated with Ceratites 

 vogdesi, Anolcites zuhitneyi, Beyrichites rotelliforniis^ 

 Gymnotoceras blakei, etc. 



Trachyceras (Protrachyceras) shastense Smith, sp. nov. 



Plate XLVI, Fig. g-ga; Plate XLVIII, Figs. 3-4. 



Form involute, subrobust, laterally compressed, whorls deeply embracing, 

 and deeply indented by the inner volutions. Sides flattened, venter narrow 

 and rounded, with shallow ventral furrow. Umbilicus narrow, exposing 

 only the umbilical shoulders of the inner whorls. The height of the whorl is 

 more than one-half the diameter, and the width is two-thirds of the height; 

 the whorl is impressed to more than one-fourth of its height by the inner 

 volution. The width of the umbilicus is slightly more than one-seventh of 

 the total diameter of the shell. 



The surface is ornamented with fine dichotomous falcoid radial ribs, and fine 

 spiral rows of knots on the ribs. There are about eleven of the spiral rows 

 of knots, which are rudimentary spines. As the shell grows older the 

 number of rows is augmented by intercalating secondary rows, smaller 

 than the primary. The row of spines bordering the ventral furrow is the 

 coarsest, as is the case with most of the Trachycerata. 



Trachyceras shastense is most nearly related to T. lecontei 

 Hyatt & Smith^ with which it is associated, but differs from 

 it in the more robust whorl, and coarser sculpture; the 

 radial ribs are coarser, and the spiral rows of knots are coarser 



iMs. The Triassic Cephalopod Genera of America, Professional Papers, U. S. Geol. 

 Sur. 1904, pi. 44, figs. 1-2; pi. 45, figs. 1-9; pi. 46, figs. 1-15. 



6) July 15, 1904. 



