144 



PROCEEDINGS OF THE ACADEMY OF 



[Feb., 



under the most favorable conditions and the hill-top form living under 

 the most unfavorable conditions. They were generally encountered 

 on the sides and near the top of the slope in the hills capped by the 

 broom-palm or in small, somewhat isolated hills, where in both cases 

 the moisture was somewhat deficient and the food, therefore, not so 

 plentiful as in the gullies. An average size was: height 24.5 mm.; 

 width 47 mm. ; index .52 ; mean divergence 125°. 



4. Somerset Hill-top (Plate IX, figs. 7, 8). — These are the smallest 

 form of this subspecies living at this locality, but they are simply a 

 diminutive form of the normal. The portion of the whorl next to the 

 periphery above is generally convex, and the height is slightly greater 

 proportionately to the width than in the normal forms. This makes 

 the index somewhat higher than in the normal, but not higher than in 

 the extinct forms. There are b\ whorls normally, the same as in the 

 form 2. An average size is: height 22 mm.; width 42 mm.; mean 

 divergence 115°-120°; index .525. 



InJ»« wijn 



• r.. ..i....r....i....i.i..ii...r....iiii.i — ■ ■ ■ ■ i t ■ ■ ■ l 



Fig. 6. — P. a. goniasmos. Comparison by whorls. 



A most satisfactory comparison of these four types may be had by 

 comparing their dimensions whorl by whorl, and such a comparison 



is given in the table below. 



Comparison of the Average Dimensions from Selected Typical Specimens 

 of the Four Types of P. a. goniasmos A.D.B. from Somerset Colony. 



Whorl Whl. 



1. 2. 



Type. mm. mm. 



Extinct 5 13 



Gully... 4.5 11 



Normal 4.15 10.8 



Hill-top 3.75 10 



Whl. Whl. Whl. 



mm. 



22 

 21 

 20 

 19 



mm. 



34 

 33 

 31 

 29 



Mean 



5. Height. Width. Index, diver- 



mm. mm. mm. gence. 



47 31 58 .535 115° 



45 23.6 50 .47 130° 



41 24.5 47 .52 125° 



39 



22 



42 



.525 



118° 



