292 SYSTEMATIC PALEONTOLOGY 



whorls about 2 mm. wide on the large whorls. Surface with fine, sharp 

 striae curving backwards to the band. 



The elongate, slender form of this shell and the band along the median 

 line of the whorls are sufficient characters to discriminate it from other 

 associated forms of gastropods. 



Occurrence. BEEKMANTOWN LIMESTONE (Ceratopea zone). Appa- 

 lachian Valley in Pennsylvania, Maryland, and Virginia. Natural sections 

 and poorly preserved casts of this species occur in the exposures east of 

 Williamsport and in the vicinity of Halfway, Maryland. 



Collections. Maryland Geological Survey, U. S. National Museum. 



HOBMOTOM A GRACILIS (Hall) 



Plate LV, Figs. 7, 8 



Murchisonia gracilis Hall, 1847, Pal. New York, vol. i, p. 181, pi. xxxix, 



figs. 4a-c; p. 303, pi. Ixxxiii, figs. la-b. 

 Hormotoma gracilis Ulrich and Scofield, 1897, Geol. Minnesota, vol. iii, 



pt. 2, p. 1015, pi. Ixx, figs. 18-21. 



Description (typical form). " Height 20 to 33 mm., apical angle very 

 constantly about 18. Shell small, slender; volutions about 14 in a 

 length of 30 mm. ; rounded generally with a slight angulation, on which 

 lies the band, a little beneath the middle; band seldom preserved, when 

 perfect, rather narrow, smooth, flat or faintly concave and margined on 

 each side by a delicate raised line; suture simple, deep; lines of growth 

 fine, bending strongly backward from the suture to the band, and beneath 

 this curving very strongly forward again, the whole indicating a deeply 

 notched mouth ; aperture a little higher than wide, rounded except below 

 where it is somewhat produced; inner lip reflected, forming a slightly 

 twisted and thickened columella." Ulrich and Scofield, 1897. 



Widely distributed in the United States and Canada in rocks ranging 

 from the Trenton to and through the Eichmond. 



Occurrence. MARTINSBUKG SHALE (Eden division). Fort Loudon, 

 Pennsylvania, and the west slope of Eickard Mountain, Washington 

 County, Maryland. 



Collections. Maryland Geological Survey, U. S. National Museum. 



