UPPER TRIASSIC SPECIES. 129 



axes nearly parallel to the keel of the periphery, or with but a slight 

 obliquity, those on the opposite sides often alternating. (Septa unknown.) 



Greatest diameter, 1.96 inches; breadth of umbilicus, 0.50 inch; con- 

 vexity, 0.47 inch. 



This form is quite unlike any of its associates, being rather peculiar in 

 the nature of its small flexuous costse, and the presence of a row of elon- 

 gated nodes around each margin of its very narrowly truncated and keeled 

 periphery, the nodes being arranged with their longer diameters nearly par- 

 allel to the curve of the truncated margin itself, and so disposed that those 

 on opposite sides alternate. The ridge along the middle of its periphery is 

 narrow, smooth, and not so defined as to be separated from the row of com- 

 pressed nodes on each side by a very deep furrow. 



This shell seems to be related to the St. Cassian species Ammonites Cor- 

 varensis, Laube, as represented by a side-view on plate xl, fig. 3, of Professor 

 Laube's Monograph of the St. Cassian Fossils, especially in form and the 

 nature of compressed nodes around each side of its periphery. It differs, 

 however, in having a proportionally wider umbilicus and well-defined costse 

 on the sides of its volutions. 



Whether Professor Laube's species has a central peripheral ridge or 

 carina, such as is seen in our shell, cannot be determined from his figure, 

 giving a side-view only, and he says nothing in his description on this point, 

 doubtless because his specimen does not show the outer margin of the volu- 

 tions : nor have we yet the means of making comparisons of the septa of 

 our species with that described by Professor Laube. 



The specific name of this shell is given in honor of William M. Gabb, 

 esq., late of the California Geological Survey, and now in charge of a geo- 

 logical survey of Santo Domingo. 



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

 Nevada; Upper Trias. 



9 PK 



