94 PALAEONTOLOGY OF KENTUCKY. 



front or rear. Beak arching over a high but very narrow fissure, which is gen- 

 erally closed, and shows a foramen a little above the middle, as may be seen 

 in figure 35, plate 34. 



Dorsal valve semi-elliptical, depressed ventricose ; greatest convexity about 

 the middle, from where it slopes with a gentle curve to the front and sides, 

 but flattened at the cardinal angles ; mesial fold well defined, extending from 

 beak to front, low and flattened, or even depressed in the middle ; beak in- 

 curved over a very narrow or linear area. 



Surface marked by fine radiating striae, which increase in size and distance 

 from apex to front ; there is neither bifurcation nor interpolation of striae. 



Size of this species is somewhat variable ; a specimen of average size has the 

 following dimensions : length, six lines ; height, seven lines, and width, nine 

 lines and one-half. In figure 6, plate 27, the dorsal valve is represented with 

 rounded cardinal angles, which is a mistake, caused by defects in the specimen 

 copied. The angles should be obtuse. 



Formation and Locality. Occurs in the upper strata of the Niagara formation, in the quarries east 

 of the city of Louisville, Ky. It is not very rare, though well preserved specimens are seldom found. 



Cyrtia exporrecta, var. arrecta. HALL. 



Plate XXVII., figure 21, and plate XXXIV., figure 35; plate XXXII., figures 60 and 61. 

 Spirifera (Cyrtia) exporrecta, var. arrecta, Hall. 24th Kep. N. Y. State Museum, page 183 1872. 

 Cyrtia trapezoidalis, var. arrecta, Hall. 27th Rep. N. Y. Museum, pi. 9, figures 22 and 23 1875. 



This shell resembles in most important points the preceding species, from 

 which it, however, differs in the following features ; 



1. The cardinal area of its ventral valve is more elevated, narrower in its 

 base, and never curved, but always straight, and generally leaning a little 

 towards the front. In several specimens before me, the cardinal area shows, 

 even in its upper part, a marked convexity, the beak curving really a little 

 into the mesial sinus ; but this feature may be the consequence of distortion, 

 though the shells in question do not show any indications of being subjected 

 to any compression or other violence. 



2. The excess of its height over its length is in these specimens considerably 

 greater than in the foregoing species. 



3. Its mesial sinus appears to be somewhat more angular, and proportionably 

 deeper than that of Cyrtia exporrecta. 



It is obvious that specimens exist which may, with equal right, be placed in 

 either species, but usually the species are easily separated. 



Formation and Locality. Found associated with the former in the upper strata of the Niagara 

 formation, in the quarries east of the city of Louisville, Ky. Is rather rare. 



