Clarke — The Naples Fauna. 



91 



whorl. In the final whorl this hyponomic tire is slightly elevated at the 

 edges, each of which is sharp, and is broadly convex over the middle. 

 Beneath the edges, on the lateral slopes of the whorl the surface is faintly 

 depressed into two low revolving furrows which are greatly increased in 

 depth by compression in fossilization. The form of the whorls varies very 

 slowly in all later growth-stages, that is to say, the adult form is approxi- 

 mated at a Very early stage of growth and afterward is subject to only unes- 

 sential modifications. Very little variation in form is shown among mature 

 individuals. In complete growth the number of volutions seems to not have 

 exceeded eight, and in final stages the length of the whorl section is about 

 three-eighths the entire diameter of the shell. 



An average entire specimen measures about 60 mm. in diameter. The 

 substance of the shell is very tenuous and the last whorl is rarely retained. 

 There is nothing in the general aspect of the shell to distinguish it from 

 Go/t. iynx. 



Development of Form. The Protoconch. 



The primitive shell is of small size compared with that of the other 

 species here described. This will be seen from a comparison of either side, 

 back or front views with those of other species. 

 Its form is remarkably similar to that in 

 Mantle. I'attersoni, and shows, as already 

 observed in that species, a greatest breadth at 

 the posterior or distal extremity, which ex- 

 ceeds the diameter of the first whorl. The 

 convexity decreases gradually, but, as in 

 Mantle Pattersoni,tlie first septum is passed 

 before the minimum width is attained. Thus 

 in the accompanying figures which give the 

 protoconch, as viewed from the ventral side, 

 it shows its characteristic swollen and pear- 

 shaped contour. 



Here again, as in Mantic. Pattersoni. all 

 observations indicate a slight asymmetry in 

 the protoconch with reference to the first 

 whorl. This is decidedly less than in MamMc. 

 Pattersoni, and appears to be susceptible of slight variation in different 

 • individuals. The size of the protoconch is itself variable within the limits 

 shown in our figures. 



Figures 68, 69. P>-obeloceras Lutkeri. Vertical 

 sections of young shells, showing the proto- 

 conch in position and presenting the proximal 

 surface, x 25. 



