Clarke — The Naples Fauna. 67 



Oonch section. It is in this respect that the differences are still more 

 emphasized. At the commencement of the nepionic stage the whorl section 

 in both forms is the same, but thereafter there is in 

 Mantic. Pattersoni a marked tachygenic narrowing of the 

 whorl so that at any given point in the length of the 

 conch or at any volution, the conch section of that species 

 is distinctly more sagittate or elongate-obcordate than in 

 Mantic iliynohostoma. This fact it is not necessary to 

 exemplify with statistical measurements; the accompany- 

 ing cuts serve to show it with sufficient clearness. Not- 

 withstanding the greater breadth of w horl in Mantic. 

 Figures. Theeariy rhynchostoma, there is, in the mature condition, as iust 



neanic varices, x 25. " 



observed, a more direct lateral slope and a more pronounced 

 ventral margin, the maximum width of the whorl being shown at the 

 umbilical edge. 



The ( r< rontic Stage. We have already observed that in Mantic. Pattersoni 

 the progressive narrowing of the whorl section eventuates in a slight depres- 



38 



Figures 36-38. M*ntieoctra$ rhynchotfma. Outlines of young shells, viewed from in front. X 4. 



Fig. 3C, 3>£ volutions; fig. 37, 3% vols. ; fig. 38, 4 vols. 



41 



40 



39 A \ / \ 



Figures 39-41. Mantic ocsras rhynthottoma Vertical sections of the early whorls, x 4. 



sion of the lateral slopes and the projection of the venter. This venter itself 

 becomes narrower and sharper with increasing age, and it is reasonably prob- 



