PRODUCTUS 103 



length of hinge-line 26 mm. The dimensions of a larger brachial valve, 

 more nearly the usual size of the species, are : length 31 mm., width 42 

 mm., concavity 8.5 mm., length of hinge-line 31.5 mm. 



The pedicle valve is regularly convex except postero-laterally, where 

 the surface towards the cardinal extremities is somewhat depressed, so 

 that the curve of the surface from the highest point of the shell to the 

 cardinal angles is at first gently convex and then concave. The surface 

 of the valve is marked by obscure, depressed, elongate nodes indicating 

 the position of spine bases, these nodes are more numerous towards the 

 front of the shell, the posterior portion being almost entirely free from 

 them in the best specimens observed. 



The concavity of the brachial valve follows rather closely the curve of 

 the pedicle valve so that the visceral cavity is very shallow, the surface of 

 this valve seems to be more completely covered with the obscure, elongate 

 spine bases than the pedicle valve, and it is also marked by faint concen- 

 tric lines, especially towards the cardinal line. 



Remarks. This species is one of the more common forms in the yellow 

 sandstone at Kinderhook, Illinois, but most of the specimens observed 

 have been somewhat weathered, so that but little more than the general 

 form and proportions can be recognized. Those specimens that do pre- 

 serve the surface characters, however, show that the shell was exception- 

 ally smooth for members of this genus, the node-like spine bases being 

 rather obscure in all cases. The species is larger than any of the hereto- 

 fore recognized Kinderhook species, and is equalled in size by but few 

 Devonian forms. It resembles P. onusta Hall from the Chemung beds of 

 New York, but is less convex, with a proportionally shorter hinge-line, it 

 also attains a larger size than that species, judging from the illustrations 

 published by Hall. 1 



Horizon. Kinderhook. 



Genus PRODUCTUS Sowerby 



Description. Shells varying in size from small to very large, deeply 

 concavo-convex in form, usually produced anteriorly, the hinge-line 

 straight and usually equaling the greatest width of the shell, commonly 

 auriculate at the cardinal extremities. Pedicle valve without cardinal 

 area or hinge-teeth, the two valves held together at the cardinal margin 

 by the strongly incurved beak of the pedicle valve. Brachial valve more 

 or less concave, or nearly flat in the visceral region, becoming more 

 strongly curved towards the anterior and lateral margins ; cardinal area, 

 dental sockets and socket plates absent or very rudimentary, the cardinal 

 process large and strong, extending far beyond the hinge-line into the 



iPal. N. Y., vol. 4, pi. 26, figs. 29-42 (1867). 



