24 A CONTRIBUTIOX TO THE LoWER DeVOXIAN FauXAS OF MARYLAND 



Description. — " Shell transversely elongated ; anterior extremity con- 

 tracted; umbones very depressed; posterior slope convex, without a de- 

 fined ridge; extremity somewhat acutely rounded. Surface marked by 

 rounded concentric striae or ridges, which are more prominent on the 

 posterior half of the shell." Hall, 1859. 



Occurrence. — Helderberg Formation, ]S[ew Scotland ^Member. 

 Devil's Backbone. 



Collection. — Maryland Geological Survey. 



INCERTAE SAEDES 



Genus ILIONIA Billings 



Ilioxia sixuata (Hall) 

 Plate LXXVIII, Figs. 9, 10 



Anatina ? sinuata Hall, 1859, Nat. Hist. N. Y., Pal., vol. ill, p. 265, pi. xlix, 

 figs. Sa-d, 1861. 



Description. — " Shell thin, equivalve, compressed, inequilateral, sub- 

 rhomboid, with the posterior side much wider than the anterior ; posterior ? 

 rounded, much compressed near the extremity, with a distinct shallow 

 groove extending from near the beak obliquely to the jjostero-basal margin, 

 and a second broad groove extending from the hinge-line, immediately 

 behind the beak, vertically to the base of the shell, each one producing a 

 slight sinuosity in the margin of the shell : umbones vertical, or not per- 

 ceptibly inclined to either side of the shell ; anterior cardinal slope nearly 

 flat, compressed and subalate. Surface concentrically marked with fine 

 subimbricating striae, which are undulated in passing over the depressed 

 lines on the anterior side of the shell." Hall, 1859. 



Several specimens from an unknown horizon in the Lower Helderberg 

 are referred to this species but not without doubt. All are in the form of 

 interior casts and none have the outline sufficiently complete to permit 

 restoration. Nevertheless, from the entire collection enough can be 

 gathered to show a very close resemblance to Hall's figures and descrip- 

 tion. The Maryland forms are in general much larger than those figured 

 by Hall. 



