402 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser, 



Involute, somewhat compressed laterally, high-whorled, with broadly 

 rounded flanks and venter, without any angle on either. Umbilicus com- 

 pletely closed; umbilical shoulders broadly rounded. The whorl is slightly 

 broader than high, the greatest breadth being even with the top of the inner 

 whorl. The height of the whorl is two-thirds of the total diameter. The 

 siphuncle lies a little below the center. The surface is smooth, there being 

 no ornamentation except the exceedingly fine radial striae of growth, which 

 bend backwards on the venter, forming a broad hyponomic sinus. 



The septa are sinuous, showing both lobes and saddles. The broad and 

 deep ventral saddle is divided by a narrow and shallow abdominal lobe; the 

 lateral lobe is long and rather broad; on the umbilicus there is a second 

 lateral lobe, shallow and broad. The internal septum is nearly straight. 



Relative dimensions of the adult shell: 



Diameter i-oo 



Height of last whorl 0.66 



Height of last whorl from preceding 0.46 



Width 0.70 



Involution o. 20 



Width of umbilicus 0.00 



The largest specimen found had a diameter of ninety- 

 seven millimeters; the average size of several mature speci- 

 mens was about seventy millimeters. 



In the youngest stages there is no lobe nor saddle, the 

 septum being straight, and the shape is globose. At this 

 stage the shell corresponds to Coloceras Hyatt {Nautihis 

 glohaius Meek & Worthen) of the Carboniferous. At 

 the diameter of seven millimeters, the ventral lobe begins 

 to develop, and at ten milHmeters the lateral lobes and 

 saddles are visible. At this stage the whorl ceases to be 

 subglobose, and becomes higher. At twenty-five millimeters 

 the shell has all the characters of maturity, and from then 

 on changes only in size. 



The ontogeny of this species enables us to connect a 

 highly specialized Mesozoic group with the Paleozoic 

 radicle. 



Horizon and locality. Upper Trias, Karnic stage, zone 

 of Trofites subbtillatus, Shasta County, CaHfornia, on the 

 divide between Squaw Creek and Pitt River, about six 

 miles northeast of the Bully Hill Mine, near the trail over 

 Brock Mountain. It was first found in this same horizon 

 near Hallstatt in the TyroHan Alps, associated with a very 

 similar fauna. 



