Maryland Geological Survey 333 



Dorsal valve plano-convex to depressed convex; beak straight and ex- 

 tending to the hinge-line. A medial sinus extends from the beak to the 

 base, which is narrow at the beak and gradually wider anteriorly and con- 

 tains three or four radiating ribs. A slight emargination is produced 

 where the dorsal sinus meets the ventricose portion of the ventral valve at 

 the base of the shell. Surface of both shells marked by strong radiating 

 simple rounded I'ihs which extend from the beaks to the base of the shell 

 and increase by implantation and interstitial addition, and are crossed by 

 faint concentric lines of growth. 



This form is not easily confused with any other species. 

 Dimensions: Length 16.5 mm.; width 14 mm. 



Occurrence. — Helderberg Formation, Keyser Me]\iber. Cumber- 

 land, Maryland; Keyser, West Virginia. 

 Collection.— 1]. S. National Museum. 



[Maynard.] 

 Genus HIPPARIONYX Vaiuixem' 

 HipPAEioxYX PROXIMUS Vauuxem 

 Plate LXI, Figs. 10-13 



Hipparionyx proximus Vanuxem, 1842, Nat. Hist. N. Y., Rept. Third Dist., 



p. 124, flg. 29, No. 4. 

 Orthis hipparionyx Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 407, pi. Ixxxix, 



figs. 1-4; pi. xc, figs. 1-7; pi. xci, figs. 4, 5; pi. xciv, fig. 4, 1861. 

 Orthis cumherlandia Hall, 1859, ibidem, p. 481, pi. xcva, figs. 20-21. 

 Strophodonta intermedia Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 482, 



pi. xcva, figs. 13, 14, 1861. 

 Hipparionyx proximus Schuchert, 1897, Bull. U. S. Geol. Surv., No. 87, p. 230 



(cites other bibliography). 

 Orthis cumherlandia Whitfield and Hovey, 1900, Bull. Amer. Mus. Nat. Hist., 



vol. xi, p. 223 (=H. proximus). 



Description. — "■ This truly remarkable impression [figured in wood- 

 cut No. 4] in no small degree resembles the under part of tlie hoof of a 

 colt, and is often taken for such." Vanuxem, 1842. 



" Shell large, subhemispherical. In youth tlie pedicle valve is very 

 slightly convex, but at maturity it is depressed, or concave, over the pallial 



' For a diagnosis, see Hall and Clarke, 1892, Nat. Hist. N. Y., Pal., vol. viii, 

 pt. i, p. 258. 



