136 PALAEONTOLOGY OF CALIFORNIA. 



face of the volutions nearly straight, the margin rounded. Sur- 

 face ornamented by large ribs arising on the margin of the um- 

 bilicus, and crossing the dorsum transversely; these ribs all show 

 a tendency to tuberculation, though alternate ones only carry the 

 tubercles well developed ; these are arranged as follows : one 

 small one on the umbilical angle, a larger one, slightly flattened 

 in the direction of the rib, and placed on the middle of the side 

 of the whorl; and a very large one, strongly flattened in the 

 same manner, and placed on the dorsal face between the median 

 line and the dorso-lateral margin ; the arrangement of the last 

 pair of tubercles is such that the dorsal face of the shell appears 

 to carry a deep and broad median groove. The alternating, 

 intermediate ribs in some cases are tuberculate, and reach the 

 margin of the umbilicus, while in others, they are nearly obso- 

 lete, and occasionally entirely wanting. 



The septum seems to be composed of a dorsal and two or 

 three lateral lobes, quite small but complex, and with large 

 saddles. 



Diameter, 3.7 inch; diameter of umbilicus, 1.4 inch; height of aperture, 1.4 

 inch ; width, 1.8 inch. 



This is one of the most strongly characterized species of the California Creta- 

 ceous, having no near allies in shape or ornamentation in the State, so far as known. 

 It belongs to the same group as A. tuberculatus, Sby., A. Michelhiianus, d'Orb., 

 and A. Archiacianus, d'Orb., three species characteristic of the Gault of Europe. 

 The first of these has a deep, narrow groove on the dorsum, and large tubercles, 

 but no transverse ribs ; the second is characterized by similar tubercles, but in the 

 place of single ribs proceeding from the umbilicus across the dorsum, the ribs bifur- 

 cate on the umbilical margin, and on the- dorsal face the tubercles are connected 

 by a series of oblique ribs, presenting a zigzag appearance. The nearer ally is A. 

 Archiacianus ; but while this resembles it in both the transverse section and in the 

 general arrangement of the tubercles, it has no ribs on either the sides or back, 

 the spaces between the series of tubercles being, in a measure, channelled. 



I take pleasure in dedicating it to my friend Dr. Ferd. Stoliczka, Palaeontologist 

 of the Geological Survey of India, in recognition of his able and valuable labors 

 among the Cretaceous fossils of that country. 



From the Shasta Group, Cottonwood Creek, Shasta County. A single specimen 

 obtained by Mr. Mathewson. 



