Monograph of the Lamellihranchiala. 



231 



Genus Cycloconcha — (S. A. IMiller). 



[Cijdoconcha, in allusion to the circular form of the shell.] 

 Shell equivalve, equilateral or subequilateral, circular or subcircular 

 in outline, and marked concentrically. Hinge line having the cardinal 

 teeth in the middle, with a lateral tooth of about the same length on 

 each side. 



Cycloconcha mediocardinalis — (S. A. Miller). 



\_Mediocardinal'u, in allusion to the position of the cardinal teeth in 

 the middle of the hinge line.] 



Fig. 21 — Cycloconcha mediocardinalis. 



Fig. 22— Hinge line and interior of riglit valve. 



Shell equivalve, slightly inequilateral, subcircular in outline, sur- 

 face smooth, with the exception of fine concentric lines of growth, 

 beaks projecting slightly beyond the hinge line, but not incurved. 

 Hinge line having one small cardinal tooth beneath the beak, deeply 

 bifid, and one lateral tooth on each side of about equal length. The 

 cardinal tooth is somewhat trigonal, slightly arcuate on the posterior 

 side and prolonged posteriorly, Pallial line is not distinct in the spe- 

 cimen examined, nor are the muscular impressions, though one may be 

 doubtfully traced at the anterior end of the anterior lateral tooth. 



Greatest length of specimen, 36-60 inch ; greatest breadth, 28'60 

 inch. 



Tlie general form of this shell has a striking resemblance to the 

 modern Cyelas. 



Found back of Newport, Kentucky, at an elevation of about 150 

 feet above low water-mark, and at the excavation for Columbia avenue, 

 in Cincinnati, at about the same elevation. As I only know of two 

 specimens having been found, and one of them a valve, both of which 

 I found myself, I am compelled at present to mark the species rare. 

 Range unknown. 



