116 PALEONTOLOGY. 



forked instead of single, the tubercles at the junction very large, and the 

 abdomen gibbous, but smooth [see figs. 6 a, pi. 11]. The terminations 

 of the pilse are at the edge of the abdomen at this stage, and show obtuse 

 tubercles, whereas in the young they are continued over the edge of the 

 abdomen to the base of the keel. This is the normal succession of these 

 characters, but variations are remarkable. Besides the earlier attainment 

 of the tubercule-shaped pilse, there are those which never have them at 

 all, and one specimen which becomes wholly smooth on the eighth or ninth 

 whorl. 



" Some specimens are also much flatter than others, and often the lines 

 of growth are so decided where they cross the siphon that at first sight they 

 may be mistaken for the pil* themselves, but a closer examination shows 

 that they subdivide the pilse. These more decided striae evidently indicate 

 arrests of growth, and are outlines of the transient mouth. If so, there 

 was a periodical pause in the building-up of the shell, as each pair of pilse 

 were about half completed. 



"Another variation occurs either through compression or subsequent 

 elliptical growth, such as is described by Barrande in Goniatites fecundus. 

 By one of these means, probably the former, in eleven specimens, a ScapMtes- 

 like shell is produced [fig. 6, pi. 11], with broad flattened abdomen [fig. 6 a, 

 same plate] and exceedingly prominent tubercles. So similar is this mal- 

 formation, that I at first considered it a true Scaphitoid, bearing to Scapliites 

 a relation similar to that of Bactrites to Baculites. 



" There is, however, not one specimen of the eleven examined which 

 is not more or less twisted laterally by compression." — (A. H.) 



Locality and position. — Cottonwood Canon, West Humboldt Range, 

 Nevada ; Upper Trias. 



Genus TRACHYCERAS, Laube. 

 Trachyc^eras Whitneyi, Gabb (sp.). 



Plate 11, figs. 3, 3 a. 



Ammonites Whitneyi, Gabb (1864), Geol. Survey of California (Palseont.), I, 23, pi. 4, 

 figs. 11 and 12. 



Shell attaining a rather large size for a species of this group, discoid 



