500 GEOLOGICAL SURVEY OF IOWA. 



PLATE VII. 



Fig. 1. The ventral valve of the same species. 



Geological formation and localities. In limestone of the age of the Ha- 

 milton group, at Clarkesville (Missouri); and in the fine sandstones of the 

 age of the Chemung group, at Burlington ( Iowa ). 



Productus snbalatus (n. s.). 



PLATE III. FIG. 10 a, b, c. 



SHELL semielliptical or semicircular, usually much wider 

 than high ; hinge extremities equalling or less than the 

 width of the shell. Ventral valve ventricose in the middle, 

 gibbous on the umbo and towards the beak, extended, flat- 

 tened and auriculate at the hinge extremities ; beak incurved 

 over the hinge line. 



SURFACE marked by fine concentric undulating strise, and 

 towards the lateral margins by strong wrinkles, and orna- 

 mented by numerous round tubular spines which are often 

 somewhat crowded near the cardinal margin on each side of 

 the beak, and sometimes arranged in a continuous line upon 

 the umbo : towards the beak the spines are smaller, and 

 become stronger towards the margins of the shell. 



The shell is often unsymmetrical, one of the ears being much more extended than 

 the other, and sometimes both are so much extended as to have an alate appearance. 

 The character of the spines, or the bases which usually remain, bear some resem- 

 blance to P. subaculeatus ; but they are less regularly disposed, and the shell is re- 

 markably distinct from any other species in our rocks, so far as I know. 



Fig. 10 o. Ventral valve of a specimen of medium size, with few spines, and preserving 

 the usual characteristics of the species. 



Fig. 10 b. Ventral valve of a larger specimen, showing the irregular distribution of the 

 spines. 



Fig. 10 c. Profile view of fig. 10 b. 



Geological formation and locality. In calcareous shales of the age of 

 the Hamilton group of New- York : Rock-island (Illinois), opposite Daven- 

 port (Iowa). 



