CRETACEOUS FOSSILS. 131 



branches are two spurs on each side; these, as throughout the 

 whole septum, are more complex ou the dorsal than on the ven- 

 tral side of the lobe. Lateral saddle similar to the dorsal, but 

 smaller. Inferior lateral lobe about as long as the dorsal, re- 

 sembling in shape the superior lateral, but not half so complex; 

 the inferior lateral branch having diminished to a mere spur. 

 Below this, to the umbilical angle, are three lobes and three 

 saddles, presenting the fundamental characters of the laterals, 



but rapidly simplifying. 



« 



This description of the septum would apply very nearly to that of A. Hoffman- 

 nii, but in that species the lobes are broader, more symmetrical on the upper and 

 lower sides, the saddles are proportionally larger, and the diminution in size and 

 complexity of the lobes from the superior lateral is more marked and rapid. 



The illustration given in Pal. Cal., Vol. 1, pi. 10, fig. 6, is of a young specimen 

 from Cottonwood Creek, Shasta County. 



Locality : Shasta Group, Cottonwood Creek. 



A. Traskii, Gabb. 



PI. 19, Fig. 7, a, b. 



(A. Traskii, Gabb; Pal. Cal., Vol. 1, p. 03, pi. 11, fig. 10; pi. 12, fig. 11.) 



Young specimens of this shell have so many points of difference from the 

 adult that they might he mistaken for a different species. The adult has a mouth 

 nearly as broad as long, the sides of the whorl being strongly arched. A specimen 

 a little over two inches in diameter, shows a section nearly identical with that of 

 A. Hoffmannii, PI. 11, fig. 13 a, while a still smaller one before me, 1.3 inch in 

 diameter, is as flat on the sides as A. Brewerii ; and the ribs, which are strongly 

 sinuous on the sides, become obsolete at the dorso-lateral margin, the dorsum being 

 very much flattened and sjnooth. 



This species is not rare at Cottonwood Creek, Shasta Group. 



A. Hoffmannii, Gabb. 



PI. 20, Fig. 8, a. 



(A. Hoffmannii, Gabb; Pal. Cal., "Vol. 1, p. G5, pi. 11, 12, fig. 13, a, b, c.) 



A variety of this shell was collected by Mr. Mathewson at Cottonwood Creek, 

 in which the whorls are much thicker than in the form described ; the aperture 



