30 Trans. Acad. Sci. of St. Louis. 



Remarhs. In general form this species is scarcely separa- 

 ble from M. chemungensis Hall, from the Chemung Group of 

 New York, but it differs from that species in its surface 

 ornamentation, there being no radiating striae upon the New 

 York species. It also approaches M. blairi M. & G., from 

 the Chouteau Limestone near Sedalia, Missouri, but seems to 

 be distinguished from that species by its more conspicuous 

 lines of growth and by its straighter and sometimes sinuous 

 basal margin. With sufficiently well-preserved material it is 

 probable that the Northview specimens would have to be con- 

 sidered as a new species, but for the present it seems best to 

 leave them without a specific designation. 



Cardiopsis radiata Meek and Worth en. 



PI. III. f. 5-6. 



Shell obliquel}^ subovate in outline, attaining a height of 

 22 mm. in the largest specimen examined, very gibbous, the 

 two valves equal, beaks prominent, elevated above the hinge- 

 line and closely incurved, directed anteriorly. Anterior and 

 basal margins regularly rounded, the posterior margin 

 obscurely subangular at its most posterior point. Hinge- 

 line less than the greatest length of the shell, slightly arcu- 

 ate. Surface of the shell covered with numerous fine, rounded, 

 radiating costae, which are somewhat coarser upon the pos- 

 terior umbonal slope. Between eighty and ninety costae may 

 be recognized upon the most perfectly preserved specimen. 



Remarks. Several genera have been established for shells 

 of this general form, Cardiopsis by Meek and Worthen, in 

 1861, Dexiobia by Winchell in 1863, and Oracardia by Her- 

 rick in 1888. The types of all of these were found in the 

 same general horizon, near the base of the Carboniferous 

 System in Indiana, Iowa and Ohio. Dexiobia and Oracardia 

 are both described as inequivalve, while in Cardiopsis the 

 two valves are said to be equal. Among the Northview 

 specimens no difference has been detected between the right 

 and left valves, although both have been studied, and for 

 this reason, chiefly, they are referred to the genus Cardi- 

 opsis. The presence or absence of hinge-teeth in these 

 specimens cannot be determined. In the original generic 



