Maryland Geological Survey 599 



of Town Creek, 1723 ; Town Creek, 18G3; 2 miles west of Pawpaw, West 

 Virginia, 1704 abundant; 2i/2 miles north of mouth of Sideling Hill 

 Creek common. 



Collection. — Maryland Geological Sun'cy. 



Spirieer mucronatus var. rosTERUS Hall and Clarke 

 Plate LVIII, Figs. 1-10 



Delthyris mucronata Hall, 1843, (in part), Geol. N. Y., Rept. 4tli Dist., p. 



270, fig. 3. 

 Spirifer mucronatus var. posterns Hall and Clarke, 1893, Pal. of N. Y., vol. 



viii, pt. 11, p. 361, pi. xxxiv, figs. 27-31. 



Description. — Shell semicircular to triangular ; cardinal angles mu- 

 cronate, extremities often greatly extended; hinge-line attaining a length 

 of two, three or more times that of shell ; sides straight or curving, usually 

 oblique, or occasionally nearly at right angles to hinge-line. Hinge-line 

 straight. Area narrow triangular. 



Ventral valve often but little more convex than dorsal, in other speci- 

 mens more gibbous. Umbo small, incurved over area. Mesial sinus 

 broad in front, extending to apex, limited by two plications which may be 

 more prominent than others. Sinus deep, often quite wide in front, in 

 some specimens simple, in others bearing a low plication in center, the 

 position of which is indicated in a few individuals simply by a retral 

 curvature of the laminae. Dorsal valve usually moderately convex; fold 

 prominent, often wide in front, bearing, in most individuals, a median 

 groove. 



Surface ornamented by about 10 to 15 rather coarse angular plications 

 which are obsolete on the mucronate extensions; plications crossed by 

 numerous imbricating lamella? which are often crowded in front. Young 

 specimens, when very well preserved, are seen to be ornamented by fine, 

 interrupted, radiating striae. These striae are but faintly visible in adult 

 specimens and are seen only in unusually well-preserved individuals. 



Interior of ventral valve bears short teeth, scarcely noticeable in cast, 

 and a striated muscular area in the center of which are small elongate 

 scars for the attachment of the adductor muscles. Median septum absent 

 or represented by a very faint line. 



