PALAEONTOLOGY OF IOWA. 499 



The prevailing character of the species, in its surface markings, is that of a con- 

 centric imbricating or larnellose structure; while in some specimens we have, in 

 addition to this, the elongate spiniferous tubercles towards the umbo, and elongate 

 ridges in the lower part of the shell. 



There is no visible area in several specimens examined, when the two valves and 

 hinge line are entire; and the interior of the dorsal valve shows a strong bilobed 

 cardinal process, without sockets for the cardinal teeth of the opposite valve as in 

 true STROPHALOSIA. 



Fig. 8 a. Ventral valve, showing a few spiniferous tubercles on the upper part of the 

 valve, while the ridges which usually characterize the lower part in such spe- 

 cimens are not perceptible. This specimen is proportionally narrower than the 

 prevailing forms. 



Fig. 8 b. Ventral valve of the ordinary form, without spiniferous nodes or ridges. 



Fig. 8 c. Dorsal valve of another individual, with some undefined radiating ridges upon 

 the surface. 



Fig. 8 d. Profile view of a specimen. 



Fig. 8 e. Interior of a dorsal valve, showing the cardinal process, muscular impressions, 

 etc. 



Geological formation and localities. In calcareous shale and in limestone 

 of the age of the Hamilton gi*oup : Hamburgh (Illinois), and Louisiana 

 ( Missouri )*. 



Productus slmniardianus ( n. s.). 



PLATE III. FIG. 9; and PLATE VII. FIG. 1. 



SHELL semielliptical, wider than long ; hinge line about 

 equal to the width of the shell. Ventral valve very convex, 

 gibbous in the middle and towards the umbo ; beak incurved. 

 Dorsal valve moderately concave ; extremities flattened, 

 subauriculate. 



SURFACE marked by fine concentric striae and strong spi- 

 niferous tubercles, which are sometimes crowded near the 

 cardinal extremities of the ventral valve. 



This species has been referred to P. subaculeatus and P. murchisoniamis ; but it 

 differs from the former in the smaller and less rounded character of the bases of the 

 spines, and from the latter in its smaller size, less numerous and less regularly ar- 

 ranged spiniferous tubercles, and the absence of the strong radiating ridges shown in 

 the figures of the dorsal valve. 



Fig. 9. The interior of a dorsal valve, showing the marks of the spiniferous ridges upon 

 the inner surface. 



* At the time these specimens were figured, I regarded certain other forms from near 

 Burlington ( Iowa) as identical with them : a farther comparison, however, shows the 

 propriety of separating these specimens from those of that locality. 



