1G*2 PAL-EONTOLOOY OF NEW-YORK. 



The illustrations on Plate xxiu, figures 18, 19 & 20 are enlarged three diameters, and 

 those from 12-17 are enlarged two diameters. The interior of the dorsal valve show- 

 ing the cardinal process, is enlarged six diameters. 



Figure 23 represents the impression left in the stone by a dorsal valve, with the area 

 and minute foramen of the ventral valve. This one is a little less broadly truncate, 

 and presents otler slight differences from the ordinary forms of this species. 



Geological formation and localities. This species occurs principally in the calca- 

 reous beds of the Marcellus shale, and in the shale itself. It is found in largo num- 

 bers near Schoharie, and also near Manlius in Onondaga county, at Avon in Liv- 

 ingston county, and on the shore of Lake Erie above Buffalo. It occurs also in the 

 shale of the Hamilton group near Tully in Onondaga county. 



Pioductella duiiiosa. 



PLATE XXIII. 



Produclut dumotiu : Hall, Fourteenth Report on the State Cabinet, p. 99. 18G1. 

 Compare Producttu ipinulicosta .- Hall in Tenth Report on tlie State Cabinet, p. 173. 1857. 

 Compare Productu* thumardianui : Geol. Report of Iowa, Vol. i, part ii, pa. 499, pi. 8, f. 9, and pi. 7 



f. 1 ; and this volume p. 150. 



Body of the shell subovate, and, including the ears, somewhat hemi- 

 spheric ; length and width about equal ; the hinge-line usually a little 

 less than the greatest width of the shell. 



Ventral valve extremely gibbous or ventricose in the middle, contracted 

 at the umbo, which is prominent, with the beak strongly incurved 

 over the hinge-line. The middle and lower part of the shell is regu- 

 larly arching, and almost vertically depressed at the sides of the umbo 

 to the narrow flat ears. Dorsal valve more or less deeply concave, cor- 

 responding nearly with the opposite valve. 



The surface of the ventral valve is marked by numerous slender spines, 

 the bases only of which are usually preserved. The spines on the umbo, 

 and a row on the ears, rise almost vertically from the surface ; while 

 below this the surface is marked by short rounded ridges, each of 

 which terminates in a slender spine. Towards the front of the shell 

 the ridges are continuous, being only slightly interrupted by the 

 growth of the spines. The bases of the spijfes are more or less distinctly 

 arranged in concentric rows, and vary greatly in their number and 

 proximity to each other. The entire surface is marked by closely 



