156 PROCEEDINGS OF THE ACADEMY OF [Feb., 



field stations, but the old high-spired form shown on the Bloomfield 

 diagram in the extinct shells is probably here found surviving in a 

 more diminutive form. It is a hill-top race, for the King Edward 

 woods is all hill-top, while the neighboring Benmore woods is cleared 

 at the hill-top and the Cedar Hill woods includes hill-top and slopes 

 and is much more wet than King Edward Hotel woods, so that stunt- 

 ing due to dry conditions is not so operative there. The species is 

 still very plentiful here, another indication of adjustment of the forms 

 to their environment. But, if such. an adjustment as seems to be 

 indicated has really taken place, it must have been accomplished 

 since the isolation of this patch of woodland by the encroaching 

 cultivation; that is, since the settlement and clearing of the adjoining 

 lands, and probably within 20 or 30 years. That such change of 

 conditions due to the influence of man in clearing the forest has re- 

 sulted in smaller races in some species is easily demonstrated in the 

 case of other Jamaican land mollusca, and some examples of it I 

 intend to consider in a subsequent paper. 



A comparison of the forms from these nine colonies, including the 

 four types described from Somerset, whorl by whorl, as was done in 

 the case of Pleurodonte bainbridgei from the five colonies compared 

 under that species, shows a very definite gradation in the dimensions. 

 For the purpose of this comparison a small number of specimens, 

 five to ten, chosen as typical examples of each type or colony and 

 representing the variation in size, were selected, making twelve groups 

 of specimens, or some 75 specimens in all. Each one of these was 

 then measured for its diameter at the end of the first, second, third, 

 fourth and fifth whorl, also for its greatest diameter and height. The 

 number of whorls and fractions of a whorl were noted. In measuring 

 the diameters at the end of the first, second, etc., whorls all measure- 

 ments started at the protoconch origin. In the larger snails the 

 beginning of the suture is generally straight for half a millimeter 

 or more and this straight part prolonged becomes a tangent to the 

 curving suture line. From the beginning of the suture a line was 

 drawn radially, and tangent to the first part of the suture ; where this 

 line crossed the suture was taken as the end of the whorls. The 

 series of measurements thus obtained are strictly comparable, being 

 all made on the same basis. With the heights and greatest widths 

 they amounted to some 500 measurements in twelve series; and in 

 nearly all cases represent 7 measurements for each shell. These were 

 then taken in each series or type, whorl by whorl, and the correspond- 

 ing measurements averaged, giving for each of the twelve types or 



