NATURAL SCIENCES OF PHILADELPHIA. 455 



ill the same direction. An internal cast of one of the left vales shows a large, 

 circular, muscular impression, located a little behind the middle of the valve. 



Height, 3-67 inches; breadth, 3-69 inches; convexity of left valve, 0.70 

 inch ; number of radiating costaj in 0-88 inch, at a distance of 2 inches from 

 the beak, five. 



In the shortness of its ears, the deep sinus in the anterior edge of its right 

 valve, and its surface markings, this species seems to bear close relations to 

 the typical forms of the genus Monotis, but it wants the cartilage-pit under 

 the beak, said to be characteristic of that genus, while its geological position 

 is far below the range of any of the known species of Monotis. 



Locality and position. Monroe county, Illinois. Keokuk Limestone of 

 Lower Carboniferous series. 



Aviculopecten pellucidus. Shell small, extremely thin, broad subovate or 

 subcircular, exclusive of the ears, slightly oblique ; hinge margin straight, 

 less than the greatest diameter of the valves, from the front to the posterior 

 side ; ventral margin nearly regularly rounded ; anterior edge rounded near 

 the middle, and passing by a slightly oblique curve into the ventral border ; 

 posterior margin most prominent below the middle. Left valve much com- 

 pressed ; anterior ear rather small, triangular, flattened and distinct from the 

 umbonal slope, separated from the anterior edge by a broad, subangular 

 sinus ; posterior ear about the same size as the other, flat, and terminating 

 in a rather acute angle, separated from the posterior margin by a broad, very 

 shallow, rounded sinus ; beak compressed, or but slightly convex, and located 

 near the middle of the hinge. Surface ornamented by very fine, radiating, 

 thread-like strise, which increase by implantation, and are generally rather 

 smaller than the depressions between ; crossing these are equally fine, regular, 

 less distinct, concentric lines. (Right valve unknown.) 



Diameter from ventral margin to the hinge, 0*36 inch ; breadth from the 

 posterior to the anterior side, 0-35 inch ; length of hinge, 0'23 inch ; number 

 of radiating strise in 0-10 inch, at the ventral margin, about 12. 



Locality and position. Adams county, Illinois. Coal Measures, in dark 

 bituminous shale. 



Genus AVICULA, Klein. 



Avicula oblonga. Shell small, nearly semicircular, slightly oblique ; hinge 

 straight, and longer than any other part of the valves. Left valve moderately 

 convex ; anterior wing triangular, compressed, separated by a broad, unde- 

 fined sulcus, from the swell of the umbo, very slight sinuous on its margin, 

 which intersects the hinge border nearly at right angles ; posterior wing 

 larger than the other, compressed, broadly sinuous in outline, and 

 terminating in an acute angle ; beak small, pointed, slightly oblique, in- 

 curved, and scarcely passing beyond the hinge line ; surface ornamented by 

 about twenty-six simple, sharply elevated, linear, radiating costae, separated 

 on the middle of the valve, by spaces from three to five times their own 

 breadth, and crossed by similar regularly disposed concentric lines, so as to 

 produce a neat, coarsely cancellated style of ornament. (Right valve 

 unknown.) 



Diameter from ventral margin to hinge, 0*39 inch ; breadth from the pos- 

 terior to the anterior edge, 0-60 inch ; length of hinge, 0-56 inch ; convexity 

 of left valve, 0-07 inch. 



Locality and position. Warsaw, Illinois. Warsaw Limestone of Lower 

 Carboniferous series. 



Genus MYALINA, Koninck. 



Mtalina asgulata. Shell rather large, obliquely subovate, longer than 

 high, nearly or quite equivalve, very convex near the front, cuneate poste- 

 riorly and above ; hinge margin rather long, straight, and carinated ; posterior 



I860.] 



