146 



PROCEEDINGS OF THE ACADEMY OF 



[Feb., 



than any of the Somerset shells, compared whorl by whorl. Fig. 7 

 shows the measurements of the specimens from this colony, with their 

 frequency curves of width and height. 



Fig. 7. — Somerset Road. 



Pleurodonte acuta goniasmos A. D. B. Plate X, figs. 3, 4. 



At the Ridge near Lincoln colony. 



Shell varying from the form of the Somerset normal to much more 

 elevated than those from Somerset Road and averaging higher than 

 these in index; consisting of 5 whorls or often somewhat less (4|) 

 and rarely of over 5|- whorls; with a slight concavity next to the 

 periphery above on the last whorl in most cases, but this is sometimes 

 wanting; convex below next to the periphery on the last whorl. 

 The index ranges from .50 to .634, but the average from typical 

 specimens is .565. The umbilicus is completely closed by the expan- 

 son of the lip in all of the 40 adult specimens examined. The forms 

 are both one-toothed and two-toothed in about equal proportions; 

 with two teeth there were 18 specimens, but of these 13 had the second 

 inner tooth small; in all 45 per cent, were two-toothed. Of the re- 

 maining 22 specimens 20 had only one tooth, 50 per cent. ; the other 

 two had no trace of teeth, though the lip was heavy and well developed. 

 Average width 38 mm.; height 21.5 mm.; mean divergence 112°. 

 Number of examples, 40. 



The average width is slightly less than that for the Somerset Road 

 colony, the height, on the other hand, is slightly greater; this makes 

 the average index higher, .565 for this colony as against .545 at Somerset 



