52 



Report of the State Geologist. 



ventral ridge in ephebic or gerontic stages which is early accompanied by a pro- 

 nounced recurvature of the ornamental lines, and also the slight incurvature of 

 the lateral slopes, seen in the accelerated ephebic condition of var. styliophtkim, 

 but not attained in the corresponding condition of Mantic. PaM&rsoni. 



Aperture. The progressive changes in the form of the whorl, as just 

 described, express approximately the outline of the aperture in its successive 

 phases. Its actual shape is, however, more accurately delineated by the 

 variations in concentric ornamentation, from which it becomes evident that 

 no trace of the hyponomic sinus appears until the ephebic stage is fairly 

 introduced. It becomes rapidly more pronounced in later growth, and in the 

 parephebic stage is highly developed as a deep retral - curve bounded by 

 t he sides of the broad carina, which may well be termed the hyponomic ridge. 



Umbilication. It has been incidentally remarked that, although the 

 actual size of the umbilicus regularly increases up to the full adult condition 

 of the shell, the degree of umbilication is uniformly decreasing through the 

 same period. Umbilication is due to the failure of each successive volution 



22 



Figures 15-22. Front views of entire young shells of Mantic. Pattersoni, showing the outline of the whorl section at 

 successive growth phases. All x 4. Fig. 15, 2^ volutions ; fig. 16. 2^ vols. ; fig. 17, 2% vols. ; fig. 18, 2% vols. ; fig. 19, 3 vols. ; 



fig 20, 3!4 vols. ; fig. 21, 3]4 v ° ,f : fl £- 22 > vols - 



