Maryland Geological Survey 235 



above and slightly sinuate at the base, wing comparatively large, tri- 

 angular, joining the body of the valve near the posterior extremity, de- 

 fined by the crowding and curving of the concentric striae; extremity 

 prolonged into a mucronate spine which extends beyond the posterior 

 limit of the valve. Test thin, marked by closely arranged concentric 

 striae, which at irregular intervals are crowded into fascicles, producing 

 a gently undulating surface ; on the wing the striae are closely arranged, 

 and just below the hinge-line are turned backward along the spiniform 

 extension of the wing. One specimen has a length from beak to base of 

 24 mm., height 15 mm., hinge-line about 20 mm.; small specimens are 

 often less than 10 mm. in length and height. (Hall, 1884, condensed.) 



The external impression of a left valve occurs in the Maryland Collec- 

 tion which is apparently a small specimen of this species. The end of 

 the spine is gone or else it is not as long as usual while the concentric 

 striae are rather sharper and quite similar to those on some specimens 

 of L. sociale Hall in the New York State Museum. Another specimen 

 not so well preserved was found at the same locality which Dr. Grabau 

 compared with Leptodesma rogersi calling attention, however, to the 

 fact that the shell is less oblique than in most specimens of this species. 



Length, 9 mm.; height, 6 mm. 



Occurrence. — Eomney Formation, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill. 



Collection. — Maryland Geological Survey. 



Family AMBONYCHIIDAE 



Genus MYTILARCA Hall 



Mytilarca (Plethomytilus) oviformis (Conrad) 

 Plate XXIX, Figs. 11-13 



Inoceramus oviformis Conrad, 1842, Jour. Acad. Nat. Sci., Phila., vol. viii, p. 



246, pi. 13, fig. 7. 

 Mytilarca oviformis Hall, 1870, Prelim. Notice Lamellibranchiata 2, p. 21. 

 Mytilarca {PletTiomytiliis) oviformis Hall, 1884, Pal. N. Y., vol. v, pt. i, 



Lamellibranchiata i, p. 255, pi. xxxi, ftgs. 1-S; pi. Ixxxvii, fig. 8. 



