1911.] NATURAL SCIENCES OF PHILADELPHIA. 157 



series the average width at each whorl, the average greatest width 

 and height. The same method w r as employed in measuring the 

 dimensions of P. bainbridgei shown in fig. 4. Upon plotting and com- 

 paring these measurements (fig. 6), it was seen that when the colonies or 

 types represented were arranged geographically they were at the same 

 time sorted out into two series in which the gradation of size was con- 

 tinuous from one end to the other in each. These two series are (1 ) the 

 four types from the Somerset colony, Somerset Road, Ridge near Lin- 

 coln, Garrett's woods, Bloomfield; and (2) Kendal Road, Benmore, 

 Cedar Hill woods and King Edward Hotel woods. The first has eight 

 members, from five colonies, the second represents four colonies. The 

 first group (1) represents a line from Somerset and Ridge near Lincoln, 

 approaching Mandeville from the northwest, and terminating at 

 Bloomfield on the western outskirts of the town; the second group 

 (2) reaches from Kendal Road on the northeast to the King Edward 

 Hotel woods on the east of the town. They may very well represent 

 two migration lines, as the forms probably migrated into this region 

 from the north. The two ends near the town are separated by a 

 high ridge and much cleared land, but the forms may have mingled 

 in the past. These two groups of measurements are represented in 

 fig. 6. The first at the top of the diagram is the Somerset- Bloomfield 

 group, the lower is the Kendal Road-King Edward woods group. 



The Somerset-Bloomfield Group. 



The largest form of P. a. goniasmos found in the Mandeville district 

 is the extinct race at Somerset, designated as Sm. Ex. in the diagram, 

 with an average of 5f whorls and measuring 31 x 58 mm., with an 

 index of .535. Then follows the gully forms (Sm. G.), the normal 

 Somerset forms (Sm. N.), and the hill- top forms (Sm. T.). The 

 successive whorls are marked 1, 2, 3, 4, 5; the height (Ht.) and the 

 width are marked in black dots. It will be noticed that there is a 

 gradual decrease in size in these four types in each whorl; even the 

 first whorl showing this decrease, which indicates that the dwarfing 

 begins in the egg. The irregular variation in height, and therefore in 

 index, shown in the Somerset gully form (Sm. G.) is an expression of 

 the observation that this form does not have the shape of the others 

 and is probably another race, though closely related. The diminution 

 in size for the whorls is accelerated after the third whorl and becomes 

 very pronounced in the fifth whorl, but the width varies much more 

 rapidly (as shown by the greatest width) than the height (Ht.). The 

 index of the extinct form is, however, higher than that of the hill-top 



