40 PROCEEDINGS OF THE ACADEMY 01 



gradation from the series of minute teeth into the two large ones at the ante- 

 rior end of the hiQge, the first of the smaller series immediately behind the 

 two larger ones being as minute as any of those farther back, so that the con- 

 trast between the two sets of teeth is well marked and abrupt. The hinge 

 margin was doubtless provided with a cardinal area, but as we only have in- 

 ternal cast, it has not yet been seen. 



Carbonarca gibbosa, M. and W. 



Shell transverse, short-oblong, very convex ; posterior side wider than the 

 other, and vertically subtruncated ; anterior margin rather narrowly rounded ; 

 ventral margin nearly straight along the middle, but sloping and rounding up 

 anteriorly, and more abruptly behind , cardinal edge equaling two thirds of 

 the whole lengtli ; larger anterior teeth inclined forward and upward, and 

 those of the small series ranging nearly vertically, or slightly inclined forward 

 anteriorly, and a little backward behind; umbones gibbous, but with their 

 outer and upper surfaces a little flattened, so as to impart a slightly suban- 

 gular or prominent character to the post-umbonal slopes ; immediate apices 

 of the strongly incurved beaks placed about one-fourth the entire length of 

 the shell behind the anterior extremity. Surface markings unknown. 



Length, 0-82 inch ; height to top of cardinal margin (of cast) behind the 

 beaks, 0-56 inch ; do. to top of umbones, 0-65 ; convexity of the two valves, 

 0-57 inch. 



Locality and position. Springfield, Illinois ; Upper Coal measures. Also 

 same horizon at Lasalle, 111. The specimens from the latter locality are, in 

 some examples, more depressed and oblique than the typical form from near 

 Springfield, and these may possibly belong to a distinct species, if the differ- 

 ences noted are not due to accidental distortion. If really distinct, this form 

 might be called ft depressa. 



* Macrodon delicatds, M. and W. 



Shell small, about twice and a half as long as high, moderately convex, 

 elongate rhomboidal in outline ; posterior margin obliquely truncated, so as 

 to be angular at the base; cardinal margin about three-fourths the entire 

 length, and nearly parallel with the base ; anterior extremity very narrowly 

 rounded ; basal margin nearly straight along the middle, but rounding very 

 gradually upward anteriorly ; beaks depressed, and placed about one-fourth 

 the entire length of the valves behind the anterior extremity; posterior um- 

 bonal slopes subangular from the beaks obliquely backward and downward, 

 to the sharply rounded or subangular posterior basal extremity, while the 

 spaces behind and above these slopes are compressed or slightly concave. 

 Surface marked by small ridges and lines of growth, which are crossed by 

 raised radiating lines or linear costse, rather widely separated posteriorly, but 

 more closely arranged, and less strongly defined toward the front part of the 

 valves. (Cardinal area, binge and muscular impressions unknown.) 



Length, 0-45 inch ; height, 0-19 inch ; convexity, about 0-1 1 inch. 



This little shell will be readily distinguished from its associate, formerly de- 

 scribed by us under the name M. tenuisiriatus, by its much more oblique and less 

 gibbous form ; but more especially by having its linear radiating costal sepa- 

 rated by wide intervening spaces, instead of being closely crowded together. 

 Indeed, we know of no species with which it could be confounded. 



Locality and position. Springfield, Illinois. Upper Coal Measures. 



* In the Proceedings of the Chicago Academy of Sciences for March, 1866, p. 17, we 

 suggested that in case the name Macrodon, Lycett, could not be retained for this genus, 

 because it had been previously used by Mul'ler for a genus of fishes, that it might be 

 called Parallelodon. In that case the name of this species would have to be written Paral- 

 h lotion delicatus, 



[April, 



