Geol— Vol. II. ] ANDERSON— CRETACEOUS DEPOSITS. I 25 



V 



72. Mortoniceras crenulatum, sp. nov. 

 Plate I, Figs. 17-18. 



Shell small, not above a diameter of 5 or 6 cm.; umbilicu.s wide and 

 shallow, with rounded and sloping shoulders; section of whorl quadrate, a 

 little higher than broad; surface ornamented by strong ribs, inclined forward 

 and nearly straight, with broad, round interspaces extending from within 

 the umbilicus to the keel; ribs bearing tubercules at the umbilical shoulder 

 and at the ventral shoulder, the latter extending laterally into thorn-like 

 spines. The keel is not apparently developed on the youngest whorls, 

 which are elliptical in section, but becomes visible at a diameter of about 

 4 or 5 mm. The keel, at first simple, becomes very soon finely crenulated, 

 but apparently not deeply serrate at a diameter of 5 cm. The ahell is 

 smooth up to a diameter of 2 or 3 mm. Septa not well shown. 



This shell evidently belongs to Meek's genus Mortoni- 

 ceras, but is not closely related to any other found on the 

 Pacific Coast. 



Occurrence. — Found in the lowest horizon of the Chico, 

 at Willow Creek, Siskiyou County, California. It was 

 associated with Trigonia Icana and other forms of the 

 Lower Chico below the horizon of Pachydiscus neza- 

 b err y anus. 



73. Prionotropis branneri, sp. nov. 

 Plate I, Figs. 11-16. 



cf. Prionocyclus woolgari Meek. Geol. Sur. Terr., Vol. IX, p. 455, PI. \'I1. 



Among the species that should be regarded as "repre- 

 sentative" from the Interior Basin and the Pacific Border 

 none are more worthy of prominence than the above. 



In form and ornamentation P. branneri strongly recalls Meek's species 

 from the Upper Missouri, but it is more inflated. 



Shell more or less discoidal, but not compressed; greatest diameter of 

 largest specimen found 12 cm., though fragments of larger specimens were 

 collected; thickness at this diameter, 3.5 cm. Keel simple at first, appear- 

 ing at a diameter of 2 mm., showing faint undulations at i cm., and in old 

 age breaking up into a median row of nodes with rounded outline and with 

 rounded intervening depressions; umbilicus relatively wide, equal to about 

 three-eighths of entire diameter of coil, having abrupt walls, especially at the 

 diameter of 3 or 4 cm. Ribs twenty-five in number, simple at first, appearing 

 at a diameter of 2 mm. or earlier. At 5 cm. tubercules begin to develop upon 



