70 



Report of the State Geologist. 



these continue for ;t very short distance only, not more than one-third of a 

 volution, when there directly becomes manifest a difference in prominence and 

 a tendency to group in twos of equal size, with minor intercalations. Thus, 

 w hen one volution of neanic growth is completed there is a pronounced differ- 

 ence in the varices, and though thereafter on later whorls these lamellae evince 

 a tendency to become diffuse, yet their grouping into fascicles is continued to 

 maturity and on the mature whorl produces a modification in the contour of 

 the conch, so that the umbilical edge of the whorl is palpably nodose, though 

 the striae, at this phase, are of subequal size. In the third whorl a ventral or 

 hyponomic flattening is very well defined, and though it at 

 no time materially affects the contour of the whorl, it is 

 rendered <piite conspicuous by the course of the surface striae. 

 As shown in our figures, the apertural and hyponomic 

 curvatures of these lines are very decided in the anephebic 

 condition. At full maturity, the ventral flattening is obso- 

 lescent and the curvature of the striae notably straightened, 

 leaving us to infer a considerable modification in the outline 

 of the aperture of the conch at this stage. 



This species is not of very common occurrence and has 

 been observed only in the Styliola limestone, on Canandaigua 

 lake. Its differences from other species of Manticooebas 

 here described may be thus expressed: 

 Fig ure44. .vanticoceras Mcmticocc t& » app n v 'mat wm . of the same prenuncial fauna, 

 SS^SSSS is likewise closely nmbilicated, the ornament at its intro- 

 whorls - duction is of the same character, but remains simple both 



in the primary stage, and so far as it has been traced ; the final whorls 

 are broader with a depressed rather than a convex curve, similar in this 

 respect to Mantio. Pattersoni var, styMophilwm. Manticoceras fascioulatum, 

 of the same fauna, passes through similar early stages in the development 

 of its ornamentation, but its varices are definitely duplicate for a longer 

 period: its umbilication is much greater and its whorls broad and primitive 

 in their expression. Manticoceras tardum,i8 broadly umbilicated like the last 

 named species, and the simplicity of its ornament is maintained until an 

 early ephebic stage. In Mantle. Pattersoni, there is greater umbilication 

 accompanied by a much simpler introduction of the varices, and a distinct 

 whorl section. 



