1911.] NATURAL SCIENCES OF PHILADELPHIA. 145 



These may be plotted and will be found in fig. 6 along with the 

 series from other colonies. The measurements of all the specimens 

 from the Somerset colonies are plotted in fig. 5. Here the dis- 

 tribution of the hill-top, normal, gully and extinct forms is shown as 

 regards frequency. The frequency curve for width shows distinctly 

 the separation of these four types. Thus at width 42-43 mm. we have 

 the limit of the hill-top forms and their blending into the normal 

 forms; at 48 mm. the drop in the curve shows the passage from the 

 normal to the gully forms, while between 52 and 53 mm. the extinct 

 forms begin. Their (apparent) relative infrequency is simply due to 

 the fact that comparatively few of the extinct shells were taken. In 

 this diagram all measurements are to the nearest whole millimeter, 

 the number of coincident measurements for any size of shell being 

 indicated by the small numbers enclosed in the circles, whose centers 

 indicate the dimensions. 



Pleurodonte acuta goniasmos A. D. B. Plate X, figs, l, 2. 



At Somerset Road colony. 



Shell varying from even flatter than the Somerset normal type to 

 considerably more elevated ; consisting of 5 whorls, the last whorl next 

 to the periphery convex above and more so below, but with a distinct 

 angle at the periphery; generally more elevated than in the Somerset 

 forms and with an average index of .545, which is higher than even 

 that of the Somerset hill-top shells. The index varies, however, 

 from .473 to .612, being in general higher in the smaller shells, and only 

 one of the larger specimens showing an index as high as .585; this was 

 an old, weathered shell, perhaps representing an extinct form. The 

 umbilicus is normally closed by an expansion of the lip, but in about 

 25 per cent, of the shells it was only partially closed. This is a one- 

 toothed race like the Somerset shells — 95 per cent, of the shells showed 

 but a single tooth, and in only one was there a poorly developed 

 second inner tooth. The diameter ranged from 35 to 44 mm. Average 

 dimensions were: width 38.4 mm.; height 20.8 mm.; mean divergence 

 115° (110° to 120°); average index .545. Number of examples, 21. 



This race, while smaller, closely resembles the forms from the 

 Somerset colony. They are smaller than the smallest of these, however, 

 the hill-top forms, and are somewhat higher in the spire also; the 

 average index of the Somerset hill-top type is .525 as against .545 in 

 this colony. In the young shells up to the second whorl the size is 

 about the same as in the normal Somerset shells of the same develop- 

 mental stage ; from that point on the forms from this colony run smaller 

 10 



