120 



Report of the State Geologist. 



Post-embryonic stages. The swelling of the conch with the opening of 

 the nepionic stage is abrupt and noteworthy. The neck of the protoconch is 

 very narrow, but from this point outward the shell widens, attaining greatest 

 width at about a half-volution, where it is live or six times as wide as at its 

 commencement. Thence it gradually contracts and with the completion of a 

 full volution it has not more than two-thirds of its maximum width. This con- 

 traction of diameter is followed by a more gradual increase. Thus the three 

 stages of nepionic growth are clearly evident. 



Whorl-section and umhilication. Figure 99, which is a complete vertical 

 section of a mature shell, serves to show the relatively great breadth and 

 shallowness of all early whorls. All are broad on the venter but the flatten- 

 ing of the ventral periphery is not so strongly brought out in this section as 

 in many of the young shells themselves ; the definition of this feature, however, 

 is one that is perfected with growth, becoming most pronounced at or just 

 before maturity. 



It will be observed that uml alication is broad and complete through at 

 least four volutions, that there is a rapid loss of umbilication and increase of 



overlap in the following volution and complete 

 closure with the ensuing whorl. The phenomena thus 

 shown, are a repetition on a more striking scale, of 

 those already shown to exist in Torn, uniangulare. 



Ornamentatioii. The earliest varical lines ap- 

 pear in the course of the second volution, are dis- 

 tant, simple and strong. Their course on the venter 

 before the third volution has, however, not been 

 determined. During the second volution, these vari- 

 ces become sharply elevated and sometimes strongly 

 thickened on the lateral slopes, make a broad, back- 

 ward and then a sharp forward curve, and recurve 

 deeply on the venter. The distinguishing mark of 

 these young shells is the strength of these striations 

 and their distance from each other. 



With regard to the septation of earlier stages no 

 observations have been made that are altogether 

 satisfactory. 



Tornoceras hicostatum is represented in our collections by specimens 

 from Big Sister and Farnum creeks, in Erie county, near Angola, and along 

 the shore of lake Erie in that vicinity ; also, by occasional specimens from 

 he soft shales near Son Yea, Livingston county, and at Naples. 



Figure 99. Tornoceras bicoutatum. An 

 ephebio shell sectioned vertically 

 through the protoconch, showing the 

 contour of the whorls and the final ex- 

 tinction of early umbilication. x 6. 



