518 GEOLOGICAL SURVEY OF IOWA. 



abruptly curved and almost geniculate in front; cardinal 

 extremities slightly contracted : upper half of shell marked 

 by strong concentric wrinkles and somewhat distant spini- 

 ferous tubercles ; lower half of shell marked by elongate 

 spiniferous ridges. Ventral valve unknown. 



This species differs from P. pyxidatus in the less extended hinge line, and the spi- 

 niferous and strongly wrinkled dorsal valve; while its greater concavity, narrower 

 form and less extension on the hinge line seem to offer sufficient characters to dis- 

 tinguish it from P. shumardianus . 



FL% 3. View of the interior of the dorsal valve of this species. 



Geological formation and locality. In sandstone of the age of the Che- 

 mung group : at Burlington, Iowa. 



Productus an- 11 at us (n. s.). 



PLATE VII. FIG. 4 a, b. 



VENTRAL valve much elevated, longer than wide, very 

 gibbous, extremely arcuate, the beak recurved upon itself 

 so that the hinge line is nearly opposite the centre of the 

 back of the shell ; hinge line shorter than the width of the 

 shell ; cardinal extremities produced into small angular ears. 



SURFACE marked by strong radiating costsB which bifurcate 

 upon the umbo and below, and sometimes coalesce towards 

 the base of the shell : entire surface covered by fine undu- 

 lating concentric stride, and, in the upper part, a few strong 

 wrinkles which are conspicuous on the ears and umbo. A 

 few marks of the bases of spines are noticed, but they 

 appear to have been irregularly distributed, and in one 

 specimen do not appear at all. Dorsal valve and interior 

 unknown. 



This species, in the character of its surface markings, its costae and concentric 

 wrinkles, resembles the P. semireticulatus : it is, however, a much smaller shell, 

 the costae are more slender and the hinge line less extended, while there is no flat- 

 tening or depression of the centre of the valve. 



Fig. 4 a. View of the ventral valve. Fig. 4 b. Profile view of the same. 



Geological formation and locality. In limestone below" the Burlington 

 limestone of the age of the Chemung group : Burlington, Iowa. 



