INVERTEBRATES. 341 



shell, owing to the prominence of the front below the beaks. 

 Cardinal area with three or four linear, longitudinal cartilage 

 furrows, one of which seems to have been deeper and wider 

 than the others near the beaks. Posterior wing slender, rather 

 compressed, and extending at least as far back as the extrem- 

 ity of the valves below, from which it is separated by a very 

 profound, rounded sinus. Surface only marked by fine con- 

 centric striae of growth. Length, about 1.05 inches ; height 

 0.86 inch ; convexity, 0.52 inch. 



This species is related to P. (Monopteria) longispina (= Gervillia longispina, 

 Cox, Kentucky Geological Report, vol. 3, pi. viii, fig. 6), but differs in being 

 much less oblique, in having its anterior margin much more prominently 

 rounded beyond its beaks, which are set farther back, as well as in having the 

 posterior extremity of its valves much less produced. Judging from Prof. Cox's 

 figure, his species would seem to also have stronger marks of growth than our 

 shell, but we have seen specimens of a form from the Coal Measures of Clinton 

 county, of this State, agreeing well with Prof. Cox's figure in other respects, 

 that presented an almost entirely smooth surface. 



GerviUia auricula, Stevens {Am. Jour. Sci., vol. xxv, March, 1858, p. 265), 

 evidently belongs to this group, and seems to us to be identical with that de- 

 scribed by Prof. Cox, under the name G. longispina. 



Locality and position: Saline creek, Gallatin county, Illinois; from a ferru- 

 ginous sandy seam in shale, apparently, judging from the affinities of its fossils, 

 holding a position near or above the horizon of the eleventh bed of coal, in the 

 Kentucky section of the Coal Measures. 



Genus MYALINA, deKoninck, 1844. 



(Ann. Foss. Belg., p. 125,) 



Mtalina Swallovi, McChesney. 



PI. 27, fig. 1, la, IS, lc, Id. 



Myalina Swallovi, McChesney, I860. New species Palaeozoic Fossils, p. 57 ; also, pi. 2, 

 fig. 6 a, 66, Illustrations of same, 1865. 



Shell small, oblique, very nearly equivalve, gibbous along 

 the umbonal slopes, and presenting almost exactly the outline 



