GASTEROPODA. lo5 



Surface marked by fine, regular, transverse, arching striae, having a retral 

 curve upon the dorsum, which is traversed by a comparatively wide band, 

 inclosed between two sharply elevated striae, distant from each other about 

 one line. 



Formation and locality. In the coarse sandy shale of the Hamilton group, in 

 Schoharie county, N. Y. 



Bellerophon Thalia. 



1'LATE XXV, FIGS. 1,2,3. 



Bellerophon Thalia, Hall. Descriptions of New Species of Fossils etc., p. 32. 1S61. 

 " " Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 60. 1862. 



" " " Illustrations of Devonian Fossils : Gasteropoda, pi. 25. 1876. 



Shell ovoid or subspheroidal. Volutions rotund, the last one gradually expand- 

 ing for half its length, becoming ventricose and abruptly spreading toward 

 the aperture, which is somewhat orbicular, with a deep sinus at the ante- 

 rior margin ; umbilicus exposed. 



Surface marked by extremely fine, even, concentric striae, bending abruptly 

 backward on the dorsum, which is marked by a sulcate carina. 



This species occurs as casts of the interior, or specimens from which the 

 shell has been mostly removed by maceration and solution. Portions of the 

 shell remaining show a surface-marking similar to B. bilobatus of the Lower 

 Silurian rocks, and the shell has nearly the same form and proportions, with 

 the exception of the banded dorsum, which is distinctly limited on each side 

 by it sharply elevated line, and is always more or less well-preserved in the casts, 

 when all the other surface-markings have disappeared. In general form this 

 species bears much resemblance to B. Pelops; but the volution is more abruptly 

 expanded near the aperture; the sides are obtusely subangular, and curving, 

 into a distinct umbilicus; the dorsal band is more conspicuous and distinctly 

 sulcate or bicarinate; the sinus in the peristome is deeper and more abrupt; 

 and the stria) are more sharply curved backward on the dorsum. All the 

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