550 PROCEEDINGS OF THE ACADEMY OF [DeC, 



Polygyra indianorum Pils. Pi. XXI, figs, l, 2 (Tushkahoma), 3-8 (Limestone Gap). 



P. divesta indianorum Pils., Nautilus, XIII, p. 39. Ferrisa, Nautilus, XIV, 



p. 28 (July, 1900). 

 P. indianorum Pils., Proc. A. N. S. P., 1903, p. 200. 



This species is still known from only a small area, the localities being 

 as follows: 



Arkansas: Poteau Mts., south of Hartford Station (Gwynn P. 0.), 

 Sebastian Co., on steep slopes under stones (Ferriss and Pilsbry), 

 April 5, 1903. 



Indian Territory: Tushkahoma, Standley and Poteau (Feiriss); 

 Limestone Gap (Simpson, Ferriss and Pilsbry). 



At Limestone Gap, on the line of the M. K. & T. R. R., we found P, 

 indianorum the commonest species, though living ones were hard to 

 get. The shells are smaller than at Tushkahoma, rarely over 22 mm. 

 diam., and about 60 per cent, of the w^hole number taken have the 

 umbilicus more or less open. There is a perfect series of gradations 

 from imperforate to as widely umbilicate as P. hinneyana; only three 

 shells of those found by Pilsbry were so open as this, and as they were 

 specially looked for, the actual proportion is probably less than 3 per 

 cent. 



These umbilicate shells, taken by themselves, might be considered 

 to be P. hinneyana were it not that they connect with imperforate 

 indianorum by an unbroken series of intergrades, and moreover even 

 those most like hinneyana have the columella perceptibly more widely 

 dilated. 



It will be seen by the table of measurements that all the specimens 

 from Limestone Gap plotted together would form a curve with two 

 nearly equal, strongly marked modes at the diameters 18 and 21 mm. 

 Separated into three series according to the condition of the umbilicus, 

 it is apparent that the imperforate (typical) form is larger than the 

 perforate, being from 18 to 22.5 mm. diameter, with the mode at 21 mm., 

 while the perforate form is from 16 to 21 mm., with the mode at 18 mm., 

 and the few really umhilicate specimens are 16.3 to 16.8 mm. in diam- 

 eter. This diminution of size correllated with a perforate or umbilicate 

 axis may indicate that the Limestone Gap race is either undeveloped or 

 retrogressive, assuming maturity while yet retaining a character of the 

 stage of youth in the unclosed axis ; the best nourished (largest) indi- 

 viduals attaining the normal closed umbilicus, while in those less favored 

 the closure is imperfect, and in only the most stunted snails is the umbili- 

 cus open. 



A somewhat different view would be that the Limestone Gap series 

 is in process of diverging to form two species, one larger and imper- 



