138 PROCEEDINGS OF THE ACADEMY OF [Feb., 



Pleurodonte bainbridgei Pfr. 



At the King Edward woods colony. 



Shell depressed, consisting of somewhat less than 5 whorls (4f to 

 4£), the last whorl rounded or slightly angled at the periphery and 

 with little or no depression at the suture, except at the end of the last 

 whorl in some cases; umbilicus closed by an expansion of the lip. 

 The single (outer) tooth is very small or nearly obsolete. Average 

 dimensions are: width 50.6, height 26, mean divergence about 140°, 

 index .513. 



S«iF, 



y ■ ■ 4,i ■ y i Tr 



_l I 1 J l I ' ■ ■ I ■ ■ ■ ■ I ■ ' ' ■ I ■ ■ ■ L_l 1— 1 1 I I I I I I M i l 



Fig. 4. — P. bainbridgei. Comparison by whorls, width and height. 



Fig. 4 gives a comparison of the average sizes of this species from 

 five of the stations where collections were made, and from the Somerset 

 estate I have plotted the dimensions of the extinct shells as well as 

 those now living. They are marked Sm. F., the Somerset extinct; 

 Sm. N., the Somerset normal living forms; R. nr. L., the forms from 

 Ridge near Lincoln; G. W., the Garrett's woods colony; Ben., the 

 Benmore woods forms; and C. H., the forms from Cedar Hill. Meas- 

 urements were taken of each shell, whorl by whorl, with the height 

 and width, and these were then averaged and the index calculated, 

 this being the ratio of the height to the width. These dimensions 

 are plotted, the diameter at whorl 1, whorl 2, etc., being marked 

 1, 2, 3, 4, 5; and the greatest width is indicated by the black dots 

 marked "width." The height of the adult shell, in the same way, is 

 indicated at H. The indices are marked "Index." It will be noted 

 that the Somerset forms have a high index and, as noted, resemble the 

 forms that have been 'named P. b. pretiosa by Adams. The five 

 localities represented on this figure are arranged geographic alky from 

 Somerset to Benmore woods and Cedar Hill woods, and it will be 

 noticed that there is a fall in the index from one locality to the 

 other, but a rapid fall from the Somerset station to the Ridge near 

 Lincoln station. This is accompanied by an actual as well as a rela- 

 tive drop in height of the shell, so that the height curve resembles the 

 index curve, but this is natural as the width curve shows less variation. 

 The shells from the Benmore woods colony were not very healthy- 



