558 PROCEEDINGS OF THE ACADEMY OF [DeC, 



Oastrodonta demissa brittsi (Pils.). 



Zonites brittsi Pils., Nautilus, V, p. 99, 1892 (Hot Springs, Ark.). 

 Gastrodonta demissa var. lamellata Pils., Nautilus, XIII, p. 107, January, 



1900 (Tushkahoma and Poteau, I. T.). 

 G. demissa and var. brittsi and lamellata Pils., Proc. A. N. S. Phila., 1900, p. 



456; 1903, p. 213. Ferriss, Nautilus, XIV, July, 1900, p. 31. 

 Zonites demissa Binn., brittsi Pils. and gularis Say, Sampson, Prelim. List 



Moll, of Ark.; Ann. Rep. Geol. Surv. Ark. for 1891, Vol. II, pp. 182, 183, 



Nos. 5, 6, 16. 

 Zonites acerra Lewis, Simpson, Proc. U. S. Nat. Mus., 1888, p. 451 (Fort 



Gibson, I. T.). 



The shell varies from imperforate to as widely perforate as G. 

 demissa; the periphery is well rounded in adults, and the basal and outer 

 walls of the aperture have a white callous lining, showing yellow outside 

 on the last third of the base, the rest of the shell being olivaceous. 

 Young shells usually have at some stage of growth an entering callous 

 lamella within the basal lip, at the position of the similar lamella in G. 

 gularis. Alt. 6.5, diam. 10 mm., whorls 6^. 



Type locality, Hot Springs, Garland Co., Ark. ; distribution, the 

 Ozark uplift in Arkansas and adjacent portions of Missouri and Indian 

 Territory. 



This form has been much misunderstood, and no fewer than five 

 names have been applied to it. It was not until the series collected 

 by us in 1903 was studied that the problem reached solution. In any 

 good series from one place, it is found that some shells possess the basal 

 lamina within the mouth, and that while these shells may be of various 

 sizes, they fall short of the maximum size of the toothless individuals found 

 with them. In the fully adult shells from any colony the lamina has 

 always been absorbed, so far as our experience goes. But specimens of 

 any size may also want the lamina; so that the lamellate stage may be 

 a transitory feature appearing sooner or later, or possibly it may not 

 be developed at all in some individuals. 



In one lot of 24 shells from Hot Springs, the largest one showing a 

 lamina is 8.5 mm. in diam., and the lamina is very low, hardly notice- 

 able. All of the 7 from this size down to the smallest (5.5 mm.) are 

 laminate, most strongly so in the youngest. None of the larger shells 

 (16 individuals, up to 10 mm. diam.) show a lamina. Fig, 2 repre- 

 sents a series of these shells, drawn to the same scale. 



The types of G. brittsi were specimens just past the laminate stage, 

 but still immature. 



In some colonies the laminate stage persists in somewhat larger 

 shells than above indicated. 



The size of the umbilical perforation varies, and we do not now think 

 that any racial distinction can well be based upon its variations. The 



