CAEBONIFEROUS SPECIES. 75 



on the gibbous visceral region of the ventral valve, and no traces whatever 

 of either on the anterior and lateral slopes, composing more than half of 

 the entire surface. The ventral valve is very gibbous, distinctly arched, 

 and much produced; the elevated umbonal region being comparatively nar- 

 row, with yertical sides, and but a faint indication of a mesial sinus. The 

 spines seem to have been stout, erect, and to have arisen very abruptly 

 from the surface, without being connected with tubercles. 



In several respects, the South American shell described by Professor 

 Derby, under the name Productus Chandlessii, in the Bulletin of Cornell 

 University, already cited, would seem, as nearly as can be determined from 

 liis figures and description, to be closely allied to the species here under 

 consideration ; but, without specimens for comparison, I am not prepared 

 to express a decided opinion respecting the relations of the two shells. 



Locality and position. — The type-specimens of . this species were brought 

 by Colonel Simpson from a locality southeast of the Great Salt Lake, Utah 

 (latitude 40° 22' N., longitude 111° 38' W.), where they were found in a 

 black bituminous limestone of Carboniferous age. 



Pkoductus subhorkidus, Meek. 



Plate 7, figs. 3, 3 a, 3 &. 



Shell small, or of moderate size, generally much produced, longitudinally 

 oblong-suboval, the length being distinctly greater than its breadth ; hinge- 

 line about equaling the greatest breadth. Ventral valve very gibbous, espe- 

 cially in the umbonal region, with a deep mesial sinus extending from near 

 the umbo to the front, very strongly arched, the curve increasing from the 

 front to the beak, which is distinctly incurved, but scarcely passes the hinge- 

 margin ; ears small, nearly rectangular, arched, and standing out nearly at 

 right angles to the abrupt sides of the umbo ; surface smooth, excepting 

 sometimes broad, obscure, obsolescent longitudinal ridges on the anterior 

 and lateral slopes, and strong spine-bases, rather thickly scattered over the 

 whole, including the ears ; spines strong, erect, and long. Dorsal valve and 

 interior unknown. 



Breadth of a medium-sized specimen, 0.85 inch; length of the same, 

 measuring direct, 1.03 inches; measuring over the curve of the ventral 

 valve, 2.06 inches. 



