CAEBONIFEROUS SPECIES. 77 



umbonal region, behind the most gibbous part, depressed-convex, and with- 

 out traces of transverse wrinkles ; beak incurved, but not passing far within 

 the hinge-line ; most convex portion on each side of the mesial sinus prom- 

 inently rounded, with very abrupt lateral slopes. Ventral valve deeply 

 concave, but more or less flattened in the visceral region, where there com- 

 mences a broad mesial ridge, corresponding to the sinus of the other valve, 

 while a low prominence extends out from near the umbo to the sinuous part 

 of each lateral margin in front of each ear, internally showing a sudden gen- 

 iculation around the front and lateral margins, nearly at right angles to the 

 more or less flattened visceral region ; muscular and reniform scars unknown. 

 Surface of both valves ornamented by numerous fine, regular, rather ob- 

 scure, longitudinal costse, or striae, apparently generally destitute of spines, 

 though sometimes a few very scattering large spine-bases are seen on the 

 anterior slope of the dorsal valve, as well as on the ears of the same, where 

 they, on some specimens, form a row along the hinge-margin. , 



Greatest length, measuring from the most gibbous part of the umbonal 

 region to the front, about 1.90 inches ; from the strongly-incurved beak 

 to the front, 1.15 inches ; breadth, to the extremities of the ears, 1.6*4 inches; 

 convexity of the ventral valve, 1 inch ; number of surface striae in 0.10 of an 

 inch, 3 or 4. 



Of this species, there are some fifteen or twenty specimens before me, 

 in various states of preservation, all of which are remarkably uniform in 

 nearly all their characters. Its most marked features are its very gibbous 

 form, deep mesial sinus, dividing the ventral valve into two prominently- 

 rounded lobes, its somewhat depressed umbonal region without any traces 

 of transverse wrinkles, and its fine, even striae, with only a very few scat- 

 tering, large spine-bases, sometimes seen on the anterior slo]3e, and near the 

 liinge-margin of the ears. The sudden geniculation of the dorsal valve 

 around the anterior and lateral margins of the flattened visceral region, is 

 also so strongly marked as to give the internal view of this valve much the 

 form of StropJwmena rliomhoidalis (see fig. 3 e), excepting that it wants the 

 concentric wrinkles of that shell. Although there are specimens in the col- 

 lection showing this character very satisfactorily, unfortunately none of them 



