PRODUCTELL^ OF THE HAMILTON GROUP, 161 



Ventral valve gibbous in the middle, broadly truncate on the umbo, 



regularly curving to the front, abruptly depressed at the sides, and 



forming narrow flattened ears at the cardinal extremities. A narrow 



linear area, and a perceptible foramen or callosity under the beak. 



Dorsal valve moderately concave, minutely truncate at the apex, with 



a thickening of the hinge-line in well-marked specimens. 

 Surface of ventral valve wrinkled at the hinge-line, marked by a greater 

 or less number of spiniferous ridges, supporting slender spines of mode- 

 rate length. There is usually a row of two or three spines on the ears 

 just below the hinge-margin, and they are often closely arranged about 

 the limits of the truncation on the umbo, and more sparsely on the 

 middle and lower part of the valve. The dorsal valve is wrinkled along 

 the hinge-line, and the surface covered by numerous elongate spinife- 

 rous pustules, bearing, when "perfect, long slender spines. The entire 

 surface of both valves is marked by fine closely arranged concentric 

 striae. Interior of the dorsal valve wrinkled and pustulose, with a con- 

 spicuous depression at the umbo, and a short bifurcating cardinal 

 process. 



This little shell is extremely abundant in the calcareous bands of the Marcellus 

 shale the individuals sometimes lying in absolute contact or close proximity through 

 layers of considerable extent. They vary in size from the diameter of less than 

 one line to a quarter of an inch, and rarely to three-eighths of an inch. Notwith- 

 standing the great numbers of specimens, I have not yet been able to trace them 

 to any larger size than those illustrated, though they may attain greater dimen- 

 sions. In the larger specimens they very much resemble the young of Productus 

 (Prodndella) shumardianus ; but they are much more gibbous, the truncation is 

 broader, and the area more conspicuous. , 



The species described by me as Strophomena pustulosa from the Marcellus shale, 

 is apparently the dorsal valve of this species, some of the- specimens being illus- 

 trated on Plate xxiii. Some of these valves are larger than any of the ventral 

 valves which have been elsewhere seen ; but since we know no other species in 

 that association, I have referred them to the same. 



On the authority of M. De Verneuil, the S. pustulosa has been referred to the 

 Productus {Strophalosia) murchisonianus oi Bf. Koninck, which is likewise regarded 

 as identical with the species I have described as Productus shumardianus. 



[ PAI.iEONT0L00y IV.] 21 



