1911.] NATURAL SCIENCES OF PHILADELPHIA. 149 



present smaller form by being compelled to live under less favorable 

 conditions, especially since the region has been under cultivation. 

 They may have been driven to the shelter of this oasis in the cleared 

 land by the cultivation, and even represent a form that lived under 

 better conditions than those obtaining in the woods. 



Pleurodonte acuta goniasmos A. D. B. Plate X, figs. 8-10. 

 At the Bloomfield colony. 



A. Old or Semi-fossil Shells, 9 examples. — Shell with rather elevated 

 spire consisting of 5 to 5^ whorls and with, in most of the specimens, 

 a concavity above, next to the periphery on the last whorl ; in several 

 of the specimens the shell is excavated or concave above the suture 

 on the third and fourth whorls also, so that the very acute periphery 

 persists up to the end of the fourth whorl where the excavation below 

 the periphery disappears. The dimensions vary from 38 x 20.5 mm. 

 to 43.5 x 25 mm. ; the indices range from .515 to .612, but average .56. 

 With the exception of two specimens, these old shells are uniformly 

 larger than those now living in this colony. This is easily seen to be 

 due to the fact that the woods at Bloomfield are disturbed and there 

 is no possibility of migration from the lower ground as was doubtless 

 the case when these semi-fossil shells were alive. Across the road 

 from this woods the ground falls away into a long valley or pocket 

 in the hills where the conditions of food and moisture must have been 

 much more favorable when the country was uniformly forested than 

 those obtaining in the colony at present. The forms are both one- 

 and two-toothed, about equally divided, 4 have the outer tooth well 

 developed and the inner one rather rudimentary, 5 have only the 

 outer tooth. Average dimensions: width 40.5; height 22.7; index .56; 

 mean divergence 110°. 



B. Living Race, 25 examples. — Shell with rather elevated spire, 

 consisting of 5 to 5\ whorls ; almost uniformly convex both above and 

 below the periphery, which is not very sharp, in a few examples slightly 

 excavated above next to the periphery. The dimensions range from 

 35 x 21 mm. to 39 x 23 mm. ; the index varies from .53 to .625. They 

 are about equally divided between two- and one-toothed forms ; of the 

 25 specimens 12 have one tooth and 13 have two, but the inner tooth 

 is only rudimentary in about one-third of these. The size varies, on 

 the whole, considerably less than that for the old shells, but one is 

 above 38.5 mm. in width, and that was 39 mm. Average dimensions: 

 height 21 mm., width 37.3 mm., index 57 mm. ; mean divergence 107°. 



The dimensions of these two sets of specimens are given in the 



