INVERTEBRATES. 339 



its posterior margin. In its surface markings it seems to agree exactly with that 

 species, as well as with the European E. speluncaria. From the latter, however, 

 it differs much more widely in form than from E. Hawni ; while the muscular 

 impression of its right valve is much narrower than that of E. speluncaria, as 

 figured by Prof. King. 



The specimens of this form under examination, show very distinctly the 

 cast of the cartilage pit, which is deepest in the left valve, the beak of which 

 curves directly over it. (See cast of this pit represented by a promfcence 

 under the beak in fig. 13.) It differs from the cartilage pit in Avicula, in not 

 being in the slightest degree oblique. One of the writers, in proposing this 

 genus, upon some Kansas specimens, was unable to verify Prof. King's descrip- 

 tion of the nature of the cartilage pit, but the specimens now before us confirm 

 Prof. King's statement exactly, and there can be little doubt that the presence 

 of this pit is a constant character in this genus. 



Locality and position : Bridge of north branch Saline river, Gallatin county, 

 Illinois; Upper Coal Measures. 



Genus PTERINEA, Goldfuss, 1833. 



(Petref. Germ., ii, p. 133.) 



Subgenus MONOPTBRIA, M. and W., 1866. 



(^tovoc, solitary; nzspov, a wing — in allusion to its single wing.) 



Synon. — Gervillia, Cox, 1857. Kentucky Geol. Rep., vol. 3, p. 568; (notDefrance, 1820.) 

 Monopteria, Meek and Worthen, 1866. Proceed. Chicago Acad. Nat. Sci., 

 vol. i, p. 20. 



Shell aviculoid, obliquely produced and angular posteriorly, 

 and rounded in front ; subequivalve, both valves being'convex. 

 Posterior wing slender and produced, anterior one obsolete or 

 merely represented by a very small ear-like appendage drawn 

 back between the beaks in a deep lunule, so as not to be seen 

 in a side view, exactly as in the Silurian type Amphiccelia, 

 Hall.* Anterior margins without a byssal emargination in 

 either valve, but apparently a little gaping in the lunule, just 



* Amphicoelia, Hall, was proposed as a subgenus under Leptodomus, to which it is 

 not nearly allied. It evidently belongs to the Aviculidse, near Pterinea, though appa- 

 rently generically distinct. 



