74 CARBONIFEROUS AMMONOIDS OF AMERICA. 



On this specimen are seen two constrictions about two-thirds of a revolution 

 apart, thus making the resemblance to Prionoceras very striking. At this stage are 

 first seen the cross strife of growth. A continuation of the same generic stage is 

 shown on PI. XV, figs. 3 and 4, at 1| whorls. 



Dimensions. 



Millimeters. 



Diameter 1. 38 



Height of last coil 52 



Height of last whorl from the preceding 31 



Width of last whorl 1.02 



Involution 21 



Width of umbilicus 51 



The relative dimensions are nearly the same as at diameters 0.92 mm. and 1.29 

 mm., but the sutures differ slightlj-, the ventral lobe being slightly blunted, as sliown 

 on PI. XVI, fig. 1. On this specimen only one constriction was visible at diameter 

 of 0.85 mm. 



On PI. Xy, figs. 5 and 6 show a larger specimen still in the Prkmoceras stage, at 



2^ whorls. 



Dimnhiions. 



Millimeters. 



Diameter 1. 64 



Height of last whorl 60 



Height of last whorl from the preceding 40 



Width of last whorl 1. 38 



Involution 20 



Width of umbilicus .54 



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 neai-ly 



the .same as on the preceding specimens, except that the last whorl is proportionally 



broader and the umbilicus narrower. The sutures are the same as on the last 



specimen. The end of tiie Prionoceras stage is shown on PI. XV, tigs. 7 and 8. at 



2f of a whorl. 



Dimensions. 



Millimeters. 



Diameter 2. 25 



Height of last w'horl 87 



Height of last whorl from the preceding 50 



Width of last whorl 1. S2 



Involution _ 37 



Width of umliilicus 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 liecomes considerably narrower. The sutures are like 

 those of the smaller specimens, but on the last half whorl the ventral lobe becomes 

 A-ery much flattened, and at diameter of 2.2 mm. liecoiues slightly indented by 

 the lieginning of a ventral saddle, thus showing a transition to Gdniatitcs and 

 the end of the adolescent stage. No j-outhful stages larger than this were sue- 



