216 Monograph of the Lamellibranchiaia. 



9 



This species has not been observed by me below about 200 "feet 

 above low water-mark. From this point it is found throughout the 

 blue limestone to the Upper Silurian, into which it probably joasses. 

 The convex valve is a common fossil, and is found showing the interior 

 side about as frequently as the exterior. A specimen showing both 

 valves is, however, quite rare. It is very much larger in the upper 

 part of the Cincinnati Group than it is in the lower part or about the 

 city of Cincinnati. Specimens are found about Richmond, Indiana, 

 four inches or more in length, and the same may be said of it at other 

 places. 



Avicula hisjieta — (Conrad, 1842). 



Shell obliquely subrhombnidal, depressed, convex ; hinge line ex- 

 tended ; anterior wing short, obtuse or rounded ; posterior wing tri- 

 angular, acute, extending a little beyond the margin of the shell; 

 surface marked by unequal concentric striae and stronger wrinkles, 

 and longitudinally, along the middle of the shell, by obscure radii. 



It is found at all elevations about Cincinnati higher than 200 feet 

 above low water-mark. Hanng a frail shell ; good specimens are not 

 common. 



Genus 3Iodiolopsis — (Hall, 1847). 



Character, equivalve, inequilateral, elongated, becoming broader pos- 

 teriorly ; umbones near the anterior extremity, which is marked by a 

 single, strong, muscular impression, as in modiola. A sinus often 

 extends from the anterior side of the umbones obliquely backward, 

 leaving the anterior portion separated as a kind of lobe. Surface 

 marked by fine concentric striae ; shell thin. 



One of the most prominent characters is the strong muscular im- 

 pression, which is close to the anterior margin : this is often visible in 

 the shell, forming a little circumscribed elevation, and more conspic- 

 uous in the cast, where it is usually well i)reserved. There is often a 

 slight contraction or sinus below, or posterior to, the umbones, but this 

 is not always conspicuous. The shells of this genus are, for the most 

 part, smooth, or marked only by fine concentric lines, indicating the 

 laminae of the shell, and they are generally free from angular ridges. 



Modiohpsis modiolaris — (Conrad). 

 [See page 149, April No. of this Journal.] 



Modiolopsis Versaillesensis — (S. A. Miller.) 



[See page 150, April No. of this Journal.] 



