GASTEROPODA. 31 



The slight thickening and fold in the columellar lip distinguish this as a 

 Strophostylus, while there is no indication of an umbilicus. The form of this 

 shell differs little from Platyostoma lineata ; but the even convexity of the spire, 

 and the absence usually of revolving striae, are distinguishing external features. 



Formation and locality. In the Schoharie grit, at Schoharie, N. Y. 



Strophostylus varians. 



PLATE XI, FIGS. 16-31. 

 Stropkottylm varians, Hall. Illustrations of Devonian Fossils: Gasteropoda, pi. 11. 1876. 



Shell obliquely subconical, or depressed subglobose. Spire moderately ele- 

 vated ; volutions about three or four, symmetrically rounded above, and 

 somewhat gradually enlarging to the last one, which is very ventricose, 

 extending downward and forward. Aperture ovate or suborbicular ; 

 peristome entire ; the columellar lip usually expanded and spreading 

 over the umbilicus, sometimes free, and leaving the umbilicus exposed. 



Surface finely striate, with the peculiar thread-like striae visible on well- 

 preserved surfaces, while, on the weathered portions, they become lamel- 

 Lo66 ; and on some specimens the surface is marked by peculiar waved and 

 interrupted stria) (shown in fig. 18). 



This specie-, in some respects, makes a wider departure from the typical 

 forms of Stkopiiostylus than any other of the genus known to me. The peculiar 

 ami characteristic form is preserved ; the symmetrical rounding of the volutions 

 above, and the delicately formed spire, as well as the form of aperture in some 

 specimens, are all characteristic of the genus. The striae, however, have a 

 tendency to become lamellose, or interruptedly undulating; and there are 

 evidence- of irregularity of growth and indentations in the margin of the 

 peristome (indicated in figs. 24 and 25), both above and below the periphery. 

 The form of the aperture is extremely variable, from narrow elliptical to 

 broadly expanded. There is (rarely shown) a tendency in the last volution to 

 become free (fig. 23), and the characteristic plication of the columellar lip is 



