64 PALAEONTOLOGY OF CALIFORNIA. 



by numerous rounded dichotomous ribs, wbicb arise in the um- 

 bilicus, and after crossing the margin, divide into from three to 

 five branches, which cross the sides straight to the dorsum; 

 crossing it, arched slightly forwards. Occasionally a single rib 

 arises at the umbilical margin, and crosses the back without 

 brandling. 



Septum : Dorsal lobe large, divided for balf its length into two 

 complex brandies with a small tongue between them; above these 

 brandies are two large and one small side branch. (On the dia- 

 gram, Fig. 11, the terminal branch is represented broader than 

 usual, owing to the original being somewhat weathered at that 

 point.) Dorsal saddle occupying half the width of the septum, 

 deeply divided into four branches, one of which is simply notched 

 at the end; two of the others are divided into three, and the 

 remaining one into four smaller branches. Superior lateral lobe 

 long and slender, divided above the middle into one long and one 

 shorter oblique branch; the long branch is nearly straight, ter- 

 minates in a long single tooth, and has four or five spurs; the 

 short one is on the dorsal side of the lobe, terminates in two 

 points, and has two spurs on each side; above the origin of 

 these branches are two spurs on the dorsal, and one on the ven- 

 tral side. Lateral saddle about as wide as the superior lateral 

 lobe, divided into three branches at the end. Inferior lateral 

 lobe, two-thirds the length of the preceding one, with four 

 branches on each side and a terminal one. Two additional sad- 

 dles are simply dentate, and the two lobes have merely four 

 alternating branches. 



Figure 10 is of the natural size. 



From the collection of the California Academy of Natural Sciences, presented 

 by Dr. J. B. Trask. 



Locality: Arbuckle's Diggings, Shasta County. (Div. A.) 



This species is well marked, especially by the character of its septum. Exter- 

 nally, it is likely to be confounded with one of the varieties of the Protean species, 

 A. Rhnondii, of this section. From Cottonwood Creek, I have seen specimens of 

 that species, having identically the same septum as Fig. 14 a, but with ribs, only 

 differing from those of Fig. 10, in being more narrow and isolated at the um- 

 bilical margin. 



