. NAPLES FAUNA IN WESTERN NEW YORK, PART 2 33 I 



The proper designation for the latter type of shell, if obscured by 

 De Koninck, has been still further embarrassed by the employment of the 

 term Oxydiscus by Koken and by Ulrich for species with a distinctly 

 seamed back, like B. curvilineatus, though by Ulrich the name is made 

 to cover those which are seamless. 



In the American Devonic, species of this type are very rare, .Tr op. 



gilletianus Hartt and Rathbun of the Erere sandstone (Middle 



Devonic), Para, and the following, T. hyalina, being the only forms 



known. 



Tropidocyclus hyalinus sp. nov. 



Plate 18, fig. 1-4 



Shell small and delicate, coiled in one plane, volutions deeply embrac- 

 ing so that the umbilicus is quite narrow. Whorls rapidly expanding and 

 relatively increasing their dorsoventral diameter ; laterally compressed, 

 obcordate in section, rising rather abruptly from the umbilicus to their 

 greatest diameter, thence broadly sloping with slight subdorsal incurvation 

 to a narrow flattened seamless dorsum. On early whorls this section is so 

 modified that the umbilical elevation of the whorl is less and the subdorsal 

 depression greater. The subdorsal depressions are produced by two revolv- 

 ing furrows, which are much more conspicuous in early growth, giving the 

 shell an almost trilobed appearance, but become progressively obsolete 

 with age. 



The aperture is regular, not expanded, deeply emarginate on the back, 

 while projecting on the sides medially and recurved on the ventral surface. 

 The peristome is not thickened. 



Surface marked by a series of regular concentric, sharply elevated 

 angular lines and ridges following the curvature of the aperture, and hence 

 having a doubly sigmoidal curve, bending forward and becoming relatively 

 wide apart at the sides, recurving and becoming crowded on the subdorsal 

 depressions and on the dorsum, making a series of deep and narrow lingui- 

 form retral festoons, uninterrupted by revolving lines or other evidence of 

 slit band. 



