I20 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



No constrictions were visible on this specimen ; that one 

 which occurs at end of the first whorl being concealed by 

 the outer coil. The relative dimensions are nearly the same 

 as on the preceding specimens, except that the last whorl is 

 proportionally broader and the umbiHcus narrower. The 

 sutures are the same as on the last specimen. The end of 

 the Prionoceras stage is shown on PL XIV, figs. 7 and 8, 

 at two and three-fourths of a whorl. Dimensions: 



mm. 



Diameter 2 . 25 



Height of last whorl 0.87 



Height of last whorl from the preceding o . 50 



Width of last whorl i • 82 



Involution 0.37 



Width of umbilicus o . 58 



No constrictions were visible on this specimen, the earlier 

 ones being concealed by the outer whorl. The figures show 

 that the relative dimensions remain nearly as before, but the 

 umbilicus becomes considerably narrower. The sutures 

 are like those of the smaller specimens, but on the last half- 

 whorl the ventral lobe becomes very much flattened, and at 

 diameter of 2.2 mm. becomes slightly indented by the be- 

 ginning of a ventral saddle, thus showing a transition to 

 Glyphioceras, and the end of the adolescent stage. No 

 youthful stages larger than this were successfully broken 

 out in condition to figure, but the imperfect ones obtained 

 showed a gradual narrowing of the umbilicus and increase 

 in height of the whorl and involution. 



Adult Stage. 



The form of the adult Glyphioceras incisum has already 

 been sufficiently described in this paper and in Hyatt's 

 monograph. The sutures changed in increasing depth of 

 the ventral sinus and sharpening of the lateral lobes, as 

 shown on PL XV, fig. 11, taken from a specimen of diam- 

 eter of 15 mm. The early adult sutures have been figured 

 by Hyatt, Geol. Survey Texas, Fourth An. Rep., PL 

 XL VII, figs. 44-45. 



