Maryland Geological Survey 591 



Family SPIRIFERIDAE 



Genus CYRTINA Davidson 



Cyrtina hamiltonensis Hall 

 Plate LVI, Figs. 1-3 



Cyrtina hamiltonensis Hall, 1S57, 10th Rept. N. Y. State Cab. Nat. Hist., 



p. 166. 

 Cyrtina hamiltonensis Hall, 1867, Pal. of N. Y., vol. iv, p. 268, pi. xliv, tigs. 



26-33, 38-.52. 

 Cyrtina hamiltonensis Hall and Clarke, 1894, Pal. of N. Y., vol. viii, pt. ii, 



pi. xxviii, figs. 23-33, 43, 45, 46, 53. 

 Cyrtina hamiltonensis Grabau and Shimer, 1909, N. Amer. Index Foss., vol. 



i, p. 313, figs. 393. 

 Cyrtina hamiltonensis Cleland, 1911, Bull, xxi, Geol. Survey Wis., p. 75, 



pi. XV, tigs. 7-10. 



Description. — Shell small, trihedral with erect or incurved ventral 

 beak, high vertical or incurved cardinal area and elongated deltidium. 

 Median sinus and fold strongly developed; lateral slopes with G to 8 

 rounded riljs crossed by fine concentric striae. On the interior of the 

 ventral valve the dental plates along the bases of the deltidium are highly 

 developed, converge toward each other and rtnite with the sides of a very 

 strong median septum arising from the bottom of the valve. 



This species which takes its name from its occurrence in the Hamilton 

 fauna of Xew York, ranges upward in that State into the Ithaca fauna. 

 Its variations in form in both of tliese Xew York faunas are fmn^id in the 

 size of the shells, the curvature of the ventral valve and cardinal area 

 and the angularity of fold and sinus. Some shells could probably be 

 referred to the small erect shell of Xew York which Hall distinguished 

 by the vaiiclal tenn reda and which occurs in both Hamilton and Ithaca 

 faunas. 



The ]\Iaryland shells are casts shdwing the long median sei)tuni of the 

 ventral valve sujiporting a spondylium or spoon-shaped process fm-med 

 by the convergence and union of the dental plates. The cardinal area 

 is high and variously curved; the plications are from 4 to 7, those near 

 the hinge being faint; the hinge-line is straight and sometimes sliglitly 

 extended at the extremities. 



Length 15 mm. : width 15 mm. 



