88 



Report of the State Geologist. 



The Shell is deeply umbilicated, with smooth and broad whorls; the 

 latter even in the most advanced adult conditions presented by the specimens, 



strongly suggesting by their dimensions, pre-ephebic 

 conditions. The form of the whorl section through four 

 volutions is show n in the accompanying vertical section 

 through the protoconch. The specimen from which this 

 figure has been made is practically ephebic, so far as we 

 now understand the species. 



Orimmentation. A fine concentric ornamentation 

 is faintly visible over the earlier whorls but the later 

 whorls are apparently smooth. The wrinkle-layer is 

 frequently shown and bears exceedingly fine inosculating 

 striae traversing the whorls transversely. A feature 

 Figure 66. Gephyroceras cata- of marked distinction in this species is the presence 



phractum. Enlarged section 



through the protoconch. of collarets or labial ridges at irregular intervals in 



the later whorls. It is the only species of this zone in which such periodical 

 thickenings of the shell have been observed, though they are of frequent 

 appearance among the Rhenish goniatites. These collarets are sometimes 

 slight and manifest themselves principally in the irregular growth and degree 

 of overlap in the whorls. 



Protoconch and earl;/ shell stages. The protoconch has not been isolated. 

 It is, however, evidently of considerable size, distinctly protuberant beyond 

 the first whorl. It tapers rapidly from its neck, or the commencement of the 

 first whorl and at one-half revolution this whorl attains minimum width. The 

 neanic condition is smooth for one-half volution, ends abruptly in an elevated, 

 smooth varix, and is followed by striated nepionic growth. In all of these 

 respects the shell is wholly and notably similar to Manticoceras Pattersoni. 



Septation. The adult suture, that at four and one-half volutions, as 

 shown in the adjacent figure, and those also, of earlier stages, are marked by 



the large and prominent lateral saddle, situated 

 high up on the ventral shoulder. 1 'his is sub- 

 acute at three volutions, but becomes blended 

 in later growth. During the third volution the 

 ventral lobe remains very broad and undivided ; 



Figure67. Gephyroceras cataphractum . ill tile early pai't of tile foUl'tll Vollltioll, it 

 a. The entire course of the suture at three 1 • • 1 l • 1 



volutions, b. The suture at four volutions, becomes divided into short, acute ventrolateral 

 lobes and low ventral saddles. The umbilical lobe continues broad and blunt 

 in all stages. There is a narrow, but sharp dorsal lobe with a low accessory 

 lobe on the dorsal surface. 



