Il8 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



the ventral lobe becomes smaller in proportion; the coil 

 leaves its close spiral and shows decided egression, the um- 

 bilicus becomes wider, while the chamber becomes actually 

 narrower than in the Tornoceras stage, as seen from these 

 figures: width of chamber at diameter 0.74 mm. is 0.77 

 mm., at diameter 0.92 mm. it is 0.69 mm. The involution 

 also becomes less. At diameter 0.90 mm. and end of the 

 first whorl a decided constriction, marking a temporary 

 mouth of the shell, makes its appearance. This stage cor- 

 responds to the Upper Devonian and Carboniferous genus 

 Prionoceras Hyatt, ^ of which P. divisum Muenster, of the 

 Upper Devonian, and P. belvalianum de Koninck, of the 

 Lower Carboniferous, are the types. These species have 

 broad, low, rather evolute whorls, with wide umbilici and 

 smooth surfaces, ornamented only with periodic constric- 

 tions. The external sutures consist of an undivided, pointed, 

 ventral lobe, one pair of angular lateral lobes, and a second 

 pair of rounded lobes on the umbilical border. The exter- 

 nal saddles are angular and the lateral saddles rounded and 

 broad. To this genus may also be assigned Goniatites coni- 

 -pactus Meek and Worthen, Geol. Survey Illinois, Vol. V, 

 p. 611, PI. XXXI, fig. 2, of the Coal Measures; and G. 

 greencastlensis Miller and Gurley, Bull. XI, Illinois State 

 Mus. Nat. Hist., p. 44, PL V, figs. 12-14, from the St. 

 Louis group of the Lower Carboniferous. These species 

 differ from the types in the rounded external saddles, but 

 in this respect they agree with the ananeanic substage of 

 G. incisum. The beginning of the Prionoceras stage is 

 shown on PI. XIII, figs. 11 and 12, in the widening of the 

 umbilicus, egression of the spiral, and narrowing of the 

 chamber. Dimensions of the specimen: 



mm. 



Diameter o . 92 



Height of last whorl o. 33 



Height of last whorl from the preceding 0.23 



Width of last whorl o . 69 



Involution o. 10 



Width of umbilicus - o . 30 



1 '■ Genera of Fossil Cephalopods," p. 328. 



