164 PROCEEDINGS OF THE ACADEMY OF [Feb., 



environment, however, and the equilibration produces a race that 

 can live under such conditions, so that the number of individuals does 

 not diminish in a given area, but rather increases with the diminution 

 of size; by which a given food supply will serve for a larger number 

 of smaller individuals. 



Explanation of Plates VII-XI. 



Plate VII. — Pleurodonte {Eury crater a) jamaicensis Gmel. 



Figs. 1, 2. — The rather large forms from the Somerset colonies. 



Figs. 3, 4. — A larger and smaller form from the Ridge near Lincoln colony; 



these are quite as large as the Somerset race. 

 Figs. 5, 6. — The small race from Garrett's woods colon y. 



Plate VIII. — Pleurodonte bainbridgei Pfr. 



Figs. 1, 2. — The form of the shell of this species at the Somerset colonies; 

 it is the P. bainbridgei pretiosa of C. B. Adams. 



Figs. 3, 4.- — Two shells showing the form of the species at the Ridge near 

 Lincoln colony, which somewhat approaches the P. b. pretiosa of Adams. 



Figs. 5, 6. — Two shells from the Garrett's woods colony, showing the very 

 slight depression of the suture line, much less so than at the Ridge near 

 Lincoln . 



Figs. 7, 8. — Two shells from the Benmore woods colony showing the depres- 

 sion of the suture at a minimum. 



Figs. 9, 10. — Two shells from the Cedar Hill woods colony, showing their 

 close resemblance to those from Benmore woods. 



Plate IX. — Pleurodonte acuta goniasmos A. D. B. At Somerset. 



Figs. 1, 2. — The extinct race at Somerset which shows the rather high 

 spire of this type. 



Figs. 3, 4. — The Somerset gully race, with its acute periphery and mod- 

 erately elevated spire. 



Figs. 5, 6. — -The normal Somerset form found nearly everywhere except in 

 the gullies and on the hill-tops. 



Figs. 7, 8. — The Somerset hill-top form, a small race of the normal form. 



Plate X. — Pleurodonte acuta goniasmos A. D. B. The Somerset Road to 



Bloom field series. 

 Figs. 1, 2. — Forms from the Somerset Road Colony, the present race is of 



the general type of the normal Somerset forms. 

 Figs. 3, 4. — The living forms from the Ridge near Lincoln colony. 

 Fig. 5. — The extinct race at Garrett's woods colony, showing the relatively 



high spire. 

 Figs. 6, 7. — Living forms from the Garrett's woods colony. 

 Fig. 8. — The extinct race at the Bloomfield colony, showing the rather high 



spire of this type. 

 Figs. 9, 10. — Living forms from the Bloomfield colony. 



Plate XI. — Pleurodonte acuta goniasmos A. D. B. The Kendal Road to King 



Edward Hotel woods series. 

 Fig. 1. — The extinct race at the Kendal Road colony. 

 Fig. 2. — Living form of high-spire type at the Kendal Road colonies. 

 Figs. 3, 5. — Living forms of the flat type at the Kendal Road colonies. 

 Figs. 4, 6, 7. — Living forms from the Benmore woods colony. Fig. 4 is a 



two-toothed form and 6 is a one-toothed form. 

 Figs. 8, 9. — Living form from the Cedar Hill woods colony. 

 Fig. 10. — Living form from King Edward Hotel woods colony of average 



size; the highest-spired forms were found here. They closely resemble 



Pleurodonte abnormis Pfr. 

 Fig. 11. — Living form from same locality as 10, about the smallest specimen 



taken at this locality. 



Note. — All figures in Plates VII-XI are natural size. 



