G8 PALAEONTOLOGY OF CALIFOKNIA. 



Locality : The specimen figured, one of Dr. Trask's original specimens, is from 

 Arbueklo's Diggings, Shasta County. This species has been found abundantly on 

 the north fork of Cottonwood Creek, Shasta County, and the large specimen men- 

 tioned above is from Bald Hills, in the same county. It is also found at Benicia 

 and at Curry's, south of Mount Diablo. It is one of the most characteristic species 

 of the older bed* in this State. 



A. Chicoensis. 



PL 13, Pig. 17, and 17 a, b. 

 (A. Chicoensis, Trask. Proc. Cal. Acad. Nat. Sci., 1856, vol. i, p. 85, PI. 2, Pig. 1.) 



Shell discoidal, flattened. Whorls five or more, enveloping 

 about two-thirds. Sides nearly flat, slightly convex in the middle, 

 truncated abruptly on the dorsal and umbilical margins. Sides 

 marked by a series of diehotomous ribs, which originate on the 

 margin of the umbilicus, usually from a series of flattened nodes, 

 branch on or a little beyond the middle of the side, and terminate 

 at the margin of the dorsum in another row of similar nodes. 

 There are also on the ribs usually from one to four tubercles, dis- 

 tributed regularly along their length. Dorsum carinate in the 

 middle. One small specimen shows the usual form, septum and 

 dorsal carina, but the ribs and nodes on the sides and on the angle 

 of the dorsum are absent. 



Septum composed of a dorsal and three very unequal lateral 

 lobes. Dorsal lobe small, confined entirely to the flat dorsum, 

 divided by a shallow, rounded sinus, each branch terminating in 

 three or four teeth, two small spurs above. Dorsal saddle broad, 

 oblique, bifurcate. Superior lateral lobe terminating in a triden- 

 tate branch, with two large oblique spurs above. Lateral saddle 

 divided into four rounded branches. Inferior lateral lobe with 

 six small spurs. One supplementary lobe with two small terminal 

 spurs, and on the umbilical aspect of the whorl is a single tongue- 

 like process. 



The young form of this species resembles closely the corre- 

 sponding stage of A. Delawarensis, Morton, but in adult specimens 

 of that species the whorls become very thick, the ribs decrease in 



