156 Systematic Paleoxtology — ]\Iiddle Devoxiax 



Description. — Shell semielliptical ; length and Midth a? four to five or 

 eight to nine, but rarely proportionally wider. Ventral valve extremely 

 gibbons, regularly arched, the greatest elevation being about the middle 

 of the length; abruptly depressed towards the cardinal angles, which are 

 flattened, Avith the extremities deflected to the ventral side; the iimbo is 

 slightly elevated above the cardinal margin; cardinal area narrow; the 

 interior shows strong dental lamellae, a somewhat angular median ridge 

 terminates above the middle of the valve, and the surface is finely pustu- 

 lose in the middle, a little more coarsely pustulose along the deflected line 

 and nearly or quite smooth towards the margins. Dorsal valve deeply 

 concave, but not equaling the convexity of the ventral valve; cardinal 

 area more than half as wide as that of the ventral valve ; the interior sur- 

 face beyond the vascular imiH'essions is covered by elongate papillae, the 

 marks of the striae being scarcely distinct. Surface marked by from 

 twenty-six to thirty-four suljangiilar or sometimes rounded striae which 

 are often irregularly increased by bifurcation, or intercalation towards 

 the margin; in those with fewer striae, they are sharper and only half 

 as wide as the interspaces, while in those with, a larger number, the 

 striae and interspaces are equal ; there is a considerable space at the 

 cardinal angles of each valve destitute of sti'iae. Hall stated that the 

 examination of large numbers of specimens showed so many extreme 

 varieties that it appeared difficult to indicate reliable characters for 

 separating this species from C. mucronatus. The principal difference 

 appeared to be in the larger number of striae on the specimens referred 

 to C. deflecta =^ C. vicinns. 



Among the Maryland specimens of Chonetes are rather poorly preserved 

 impressions which resemble this species more closely than any other. The 

 ventral valves are gibbous, the cardinal angles flattened, the striae 'about 

 thirty-four in number, and the interior of the valves pustulose. One 

 rather large and gibbous ventral valve, which is partly exfoliated, shows 

 about 36 striae across the central part of the shell which are considerably 

 increased in number by bifurcation by the time the margin is reached. 

 On tlic well preserved portion of the shell near the umbo the radiating 

 striae are crossed bv u\imerous, fine, thread-like concentric striae, which 



