PRODUCTELL^ OF THE CHEMUNG GROUP. 183 



This species bears some resemblance to the more extreme forms of Productella. 

 shumardiana of the Hamilton group, but it is a larger and stronger shell and 

 marked by strong concentric wrinkles. The material at my command is not.sufiB- 

 cient to make full comparisons. 



Geological formation and localities. In the Chemung group, at Jasper in Steu- 

 ben county, and south of Cadiz. 



Productella bialveata (n. s.). 



PLATE XXVI. 



Shell subovate or subhemispheric ; the hinge-line about equalling, or 

 sometimes a little greater than the width of the shell below. 



Venteal valve varying from moderately convex to extremely gibbous or 

 ventricose ; the greatest elevation being in a vertical line from a point 

 just below the hinge-margin when the shell lies with the opening of 

 the valve downwards, leaving the longest curve of the outline between 

 this point and the front. The umbo is prominent, with the apex closely 

 incurved over the hinge-line. Between the umbo and the cardinal 

 angles the outline is abruptly sloping, or sometimes so depressed as to 

 leave a concave outline at the sides of the umbo. The ears are broad 

 and not strongly -defined. In well-marked specimens there is a sinus, 

 which is wider and well-defined on the lower two-thirds of the valve, 

 and can be traced to the umbo. 



DoRS.\L valve moderately concave, and marked by an elevation on the 

 lower half of its length, corresponding to the depression in the ventral 

 valve. 



The surface of the ventral valve (judging from the casts) has been essen- 

 tially free from striae, except the lines of growth. The ears are marked 

 by two or three or more wrinkles and as many spines, while the body 

 of the shell shows no indications of spines till we approach the margin, 

 where there are or two or three rows of scattered spiniferous tuber- 

 cles. The dorsal valves examined are not in a condition to enable us 

 to determine whether fossets have existed, corresponding to the spines 

 of the ventral valve. ^ 



