'M 



Pictet place A. diffi.cilis in this section, A. Brewerii is included in it also. 

 The latter pahuontologist saj^s that A. dijficilis "makes a transition " to 



the Ligati. 



Group II. — Mammillatl, Pictet. 



Ammonites Stoliczkanus, Gabb. — Variety spiniferus. 



Plate III., fig. 3, and Plate IV., fig. 1. 



A. Stoliczkann.s, Gabb. " Palaiontology of California." Vol. II., page 135. Plate 



XXIIL, figs. 16, Ida. 



fedScir:')?" , ;'fii 



Fig. 2. 



Fig. 2. — Ammonites Stoliczkanus, Gabb., var. spiniferus. Fragment showing the 

 spinous nature of the tubercles. 



Shell thick; whorls wider than high, compressed on the siphonal 

 edge and inner half of the sides; umbilicus not very large, but deep; 

 surface heavily costate ; ribs tuberculate, except on the centre of the 

 jjeriphery. 



Whorls live, increasing rather rapidl}" in size, about one-half of the 

 inner ones being ex])osed. The volutions are always broader than high; 

 in a well-preserved specimen an inch in diameter, they are distinctly 



