GASTEROPODA. 75 



Pleurotomaria rugulata. 



PLATE XX, FIGS. 1-7. 



Pleurotomaria rugulata, Hall. Thirteenth Rep. N. Y. State Cab. Nat. Hist., p. 108. I860. 



Fifteenth Rep. N. Y. State Cab. Nat. Hist., pi. 5, tigs. 4, 5. 1862. 

 " " " Illustrations of Devonian Fossils: Gasteropoda, pi. 20. 1876. 



Shell depressed-turbinate, ovoid-conical, wider than high ; spire moderately 

 elevated. Volutions four or five, very rapidly expanding, rounded exte- 

 riorly, the body-whorl forming almost the entire bulk of the shell. 

 Periphery broadly banded, and in the casts obtusely angular, especially 

 towards the aperture ; lower side very convex and abruptly depressed to 

 the umbilical area. Aperture broadly expanded, and the lip somewhat 

 reflexed on the lower side. 



Surface finely marked with closely arranged striae parallel to the lines of 

 growth, which turn a little backward from the suture, and thence in a 

 vertical direction over the convexity of the volution, curving gently back- 

 ward till they approach the peripheral band, when they are abruptly 

 deflected and gradually merged in the carina which borders the band. 

 The peripheral band is simple, comparatively wide, margined above and 

 below by a sharply defined carina, the intermediate space marked by fine, 

 curving striae. The lower side of the volution is as finely and closely 

 striated as on the upper side. The striae are often crowded in fascicular, 

 and towards the aperture form distinct folds, which affect the entire 

 thickness of the shell, producing the wrinkled aspect shown in figure 2, 

 plate 20. Similar but less conspicuous ridges often mark the whole 

 surface of the shell. The faint indication of revolving striae sometimes 

 seen, are quite superficial. 



The species usually occurs as casts of the interior, and may be distinguished 

 from the following, or other similar forms, by the low spire and extremely 

 expanded outer volution. The obtuse angularity of the periphery is not always 

 noticeable, and in such conditions it differs little, except in the elevation of 

 the spire, from casts of Platyostoma. There is also a considerable difference 



