NATURAL SCIENCES OF PHILADELPHIA. 4:! 



the posterior side of each beak, apparently to near the middle of the posterior 

 margin. Above and behind this oblique line, some three, four, or more simi- 

 lar parallel, regularly disposed lines also extend from the back part of each 

 beak to the posterior margin of the valves, thus marking off, as it were, a 

 kind of corselet, somewhat similar to what we see in Protocardia, excepting 

 that these radiating lines are scarcely visible to the unassisted eye and sepa- 

 rated by spaces many times their own breadth. In the spaces between th 

 posterior radiating lines the magnifier also shows numerous minute, raised 

 granules, but these are much larger and more prominent than, and entirely 

 distinct from, the extremely minute, crowded sculpturing seen by the aid of 

 a higher magnifier over the whole of the surface. It is probably mainly due 

 to the fact that the specimens have had the whole substance of the shell re- 

 placed by brilliant iron pyrites, that the delicate ornamentation mentioned 

 has been preserved. 



We know nothing of the nature of the hinge and interior of this beautiful 

 shell, and consequently only place it provisionally in the genus Schizodus. We 

 only know that other species found in our Coal-measures, agreeing apparently 

 in their internal characters with Schizodus, show the same kind of very minute 

 sculpturing seen on this. As some of these are very similar to the type on 

 which Mr. Conrad proposed to found his genus Prisconaia, from the Coal- 

 measures of Kansas, we requested him to examine his typical specimen to see 

 if any traces of such minute Burface markings could be seen on it by the aid 

 of a strong magnifier, and he informed us that his shell shows the same 

 sculpturing. As this marking is very peculiar, and entirely unlike any orna- 

 mentation we have ever seen on any other fossil, even in examples of various 

 types in exactly the same state of preservation, it is not improbable that our 

 shell will be found to possess the internal character of Mr. Conrad's type. 



Locality and position. From the shales over the fifth coal bed of the Illinois 

 Coal-measures, at Danville, of that State. For the use of the typical speci- 

 mens we are under obligations to Dr. J. C. Winslow, of Danville. 



Genus CLINOPISTHA, M. and W. 



Shell transversely oval, very thin, rather ventricose, equivalve, very inequi- 

 lateral; beaks near the posterior extremity and directed backward ; that of 

 the right valve with its immediate apex curving under the beak of the left, 

 which seems to be a little excavated for the reception of the same ; ligament 

 external, short, rather prominent, and occupying an oval, or lance-oval shal- 

 low cavity, formed by the slight inflection of the margins of the valves imme- 

 diately behind the beaks ; valves with their margins smooth within and closed 

 all around ; hinge apparently edentulous ; surface polished, and with merely 

 fine lines or marks of growth, sometimes crossed by very faint traces of radi- 

 ating lines, which are usually obsolete externally, but often seen on internal 

 casts ; muscular impressions shallow, those of the anterior side larger than 

 the others and subquadrangular in form, with a faint oblique ridge along the 

 upper margin ; posterior muscular impressions oval and occupying somewhat 

 flattened spaces near the truncated margin ; pallial line faintly marked and 



simple. . 



Although we have not seen very clearly the hinge of this shell, we have 

 been able, from impressions of it in the matrix, to be satisfied that it is not 

 crenated, as in the Nuculidse and Arcidx; and we believe it to be entirely 

 edentulous. In regard to the affinities of this type, however, we are left in 

 doubt. In its thinness, and the posterior position of its beaks, as well as in 

 the form and general appearance of its muscular and pallial impressions, and 

 its apparent edentulous hinge, it reminds one of Solenomya. It differs, how- 

 ever from that genus, remarkably, in its short gibbous form, rather ventricose 

 beaks, and general physiognomy, as well as in wanting the internal ridge e: 

 tending downward from the beaks, and apparently the partly internal Uga- 



1870.] 



