92 



very indefinite. Volutions three in number, or possibly four if the apex 

 were preserved, and encircled by narrow spiral ridges, which are every- 

 where crossed by numerous and crowded transverse striae. On the last 

 volution there are five spiral ridges, one at the periphery, two 'on the 

 apical and two on the basal side, but the two above the periphery are 

 much more prominent and distinct than the two which surround the 

 umbilicus. On the last volution of the spire there are two prominent 

 spiral ridges. 



Maximum breadth of the largest specimen collected, thirty-four milli- 

 metres : greatest height of the same, ten mm. 



Edge Mills, near Durham, J.f Townsend, 1879-1885 : two specimens 

 with the test preserved, and a remarkably well preserved natural mould 

 in dolomite, of the exterior of the apical side of another specimen. 



If the angulation and obtuse oarination of the periphery of the only 

 two specimens in which that part of the shell is preserved is not abnor- ^ 

 mal and due to vertical pressure, it" would seem that this species is most 

 nearly related to Polytropis alatus, the Oriostoma alatum of Lindstrom, 

 figured on Plate 16, figs. 14-19 of his monograph of the Silurian Gastero- 

 poda and Pteropoda of Gotland. The upper side of the specimen repre- 

 sented by fig. 1 on Plate 13 of the present paper is exactly similar to 

 that of the specimen of Orio^ioma angulatiim, Wahlenberg (Sp.) figured 

 by Lindstrom on Plate 20, fig. 37, of his mionograph, but in Wahlenberg's 

 species the shell is planorbiform, with rounded periphery and an extre- 

 mely wide and open umbilicus. Straparollus Niagarensis, Meek,* which 

 is probably also a Polytropis, is described as having a discoidal shell, 

 with rounded volutions and an umbilicus in which " almost the entire 

 diameter of the inner volution is exposed." 



Polytropis parvulus. (N. Sp.) 



Plate 13, figs. 10 and 10 u. 



Shell small, depressed turbinate and a little broader than high : 

 spire not much elevated, not so high as the last volution. Volutions 

 three, or perhaps four when the apex is preserved, rounded and 

 ventricose, but flattened and narrowly shouldered at the suture above : 

 suture angular but apparently not channelled : base of the last volution 

 rather narrowly but deeply umbilicated, though the inner volutions are 

 apparently covered over. 



Surface marked by narrow spiral ridges, which appear to be crossed by 

 minute and crowded transverse raised lines. On the last volution there 

 are seven spiral ridges visible (and there may be one or two more in the 



♦Report of the Geological Survey of Ohio, vol. II., pt. 2, 1875, p. 144, pi. 8, fig. 3. 



