140 PROCEEDINGS OF THE ACADEMY OF [Feb., 



conditions of food and moisture (the main controlling factors in the 

 case of this species) were most favorable, and they avoided the places 

 where conditions were unfavorable or where resistance to migration 

 was offered by the nature of the ground, such as dry, stony hills, 

 continuous cliffs and very steep slopes. Of pourse this dispersal and 

 migration was effected ages before the country was settled and cleared. 

 On the basis of these apparent migration lines, the colonies may be 

 divided into two groups. Starting in the area examined with the 

 Somerset colony, we have one group including in order the following 

 localities: Somerset, Somerset Road, Ridge near Lincoln, Garrett's 

 woods and Bloomfleld, forming a chain of colonies in one of the migra- 

 tion waves. The other may be taken as starting from Kendal Road 

 3£ miles north by east of Mandeville and includes the group : Kendal 

 Road, Benmore woods, Cedar Hill woods and King Edward woods, 

 the last three being localities to the east and southeast of Mandeville 

 Court House. From the large number of specimens collected at 

 Somerset the possibility of variation at airy one large station could be 

 studied; and as the variation noted in the different colonies depends 

 upon the same factors that control it in the case of the Somerset 

 group of colonies, a study of the species at this station will serve as an 

 introduction to the comparative study of the forms from all the 

 colonies, which will then be taken up in order as given above. The 

 migration lines can then be studied. 



Pleurodonte acuta goniasmos A. D. B. Plate VIII. 



At Somerset. 



From an examination in the field four types of the species may be 

 distinguished at this place, which may be designated as follows: 



1. Extinct forms that formerly lived in the deep gullies and honey- 

 combed rocks (localities of type A). Probably the nearest approach 

 to the original stock we have any record of. As they were only found 

 in a much weathered, semi-fossil condition, they are assumed to be 

 extinct and will be called " Somerset extinct." Their nearest descend- 

 ants are the Somerset gully forms 2. 



2. What may be called " Somerset gully forms," the forms now living in 

 localities of type A, as described under the Somerset colonies above. 



3. Somerset hill slope forms, living under the conditions described 

 under the Somerset colonies above as type B, the higher hill slopes. 

 These may be called the "Somerset normal forms." 



4. "Somerset hill-top" forms, living on hills on which the broom- 

 palm or palmetto grows (type C locality). 



