FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 95 



Genus Cyrtina. Davidson. 



Cyrtina, Davidson. Monog. Brit. Carb. Brachiopoda 1858. 

 Etymology: Kyrtia, a fishing basket. 



Shell small and resembling that of Spirifera in its general form. Shell struc- 

 ture punctate. Valves very unequal ; ventral valve elevated and pyramidal 

 in shape ; area very high, with a long narrow foramen, closed by a pseudo-del- 

 tidium ; dental lamellae converging from margins of foramen, and uniting to 

 form a mesial plate or septum, which divides the cavity of the ventral valve 

 into two compartments. Dorsal valve nearly flat or moderately convex. 

 Spires well developed, and resembling those of Spirifera and Spiriferina. 

 The genus Cyrtina is closely allied to Cyrtia, of Dalman, and Spiriferina, of 

 D'Orbigny, and has, also, near relationship with the genus Spirifera. 



Cyrtina crassa. HALL. 



Plate XIII., figures 21, 22, 23 and 24. 

 Cyrtina crassa, Hall. Pal. N. Y., Vol. IV., p. 267, pi. 271867 



Shell depressed pyramidal, semi- elliptical in a dorsal view ; hinge-line about 

 equal to greatest width of shell, with the extremities slightly rounded ; surface 

 plicate ; shell of medium size. 



Ventral valve depressed pyramidal, convex regularly arched from beak to 

 front, and also from sinus to margins of cardinal area ; sinus broad and 

 rounded in botiom ; beak extended and slightly incurved over area, which has 

 a height equal to one-half the width of shell, and which is almost straight in 

 its lower half, and regularly curved in the upper part ; fissure prominent, twice 

 as high as wide in the base 1 . 



Dorsal valve slightly convex, little curved from front to rear or in lat- 

 eral direction ; flat or even a little concave at cardinal-extremities ; mesial fold 

 broad, moderately elevated, rounded on top and strongly defined ; cardinal 

 area linear. 



Surface marked on each side of the mesial fold and sinus by about four 

 strong, low, rounded plications, which are crossed by fine, thread-like con- 

 centric striae, and a few imbricating folds. (Hall.) Size of the only specimen 

 in my possession is as follows : length, nine lines ; width, fourteen and one- 

 half lines ; and depth, nine lines and one-half. 



Formation and Locality. Occurs in the Corniferous limestone around the Falls of the Ohio, but 

 is exceedingly rare. So far as I know, only three specimens of this species were ever found ; two of them 

 belonged to the cabinet of the late Dr. James Knapp, and the third one was found by myself} it is copied 

 on plate 13, 



