FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 219 



Modiomorpha concentrica. CONRAD. 



Plato II., figures 9, 10, 11, 12 and 14. 



Plerinea concentrica, Conrad. Geol. Surv. N. Y., Ann. Rep., p. 1161838. 

 Cypricarditcs concentrica, Conrad. Geol. Surv. .N. Y., Ann. Rep , p. 52 1841. 

 Cypricardites oblonga, Ibidem. 



Modiola concentrica, Hall. Geol. Surv. N. Y., Rep. 4th Dist. 1843. 

 Modiomorpha concentrica, Hall. Pal. N. Y., Vol. 5, pt. 1, p. 2751885. 



Shell of medium size, ovate, extremely variable in its proportions ; length 

 less than twice the height ; basal margin often nearly straight, usually a little 

 concave on the anterior one-third ; posterior margin rounded abruptly below, 

 but gently curving above. Cardinal margin oblique in the prevailing forms, 

 moderately arcuate, often nearly straight ; sub-alate in many specimens. An- 

 terior end produced beyond the beaks, many specimens only slightly, abruptly 

 rounded, sometimes nasuate, limited by a broad depression extending from the 

 beak to about the anterior third of the basal margin. In our Kentucky shells 

 the shape differs as regards the anterior end, which is not at all, or at least 

 only slightly produced beyond the bend; the anterior margin is almost straight 

 or little curved, and turns at the beaks abruptly down to the basal margin. 



Valves moderately convex, but somewhat gibbous along the umbonal slope ; 

 the point of greatest convexity is about one-third of the length from the an- 

 terior margin. Hinge-line extends generally to the middle of shell, sometimes 

 even beyond the same. 



Beaks sub-anterior (in our western shells anterior), small, sharply angular, 

 appressed, directed forward. Umbonal region a more or less prominent sub- 

 angular elevation, extending obliquely from beak towards the post-basal 

 margin, but usually dying out about the middle in the length of the shell. 



Test comparatively thick, and ornamented by concentric, rounded or sub- 

 angular striae, which become lamellose and coalescing on the anterior end of 

 the valves, where they are less prominent. 



Anterior muscular impression strong, striated, situated just within the 

 anterior margin, with a small retractor scar above it. Posterior impression 

 large and shallow. Pallial line moderately impressed. Hinge furnished with 

 a strong cardinal tooth just posterior to the beak in the left valve, and a 

 corresponding depression in the right valve. No proper lateral teeth have 

 been observed, but the cardinal margin is thickened and grooved from the 

 beak backward about half the length of the cardinal line. It is impossible to 

 give in figures the dimensions of the different individuals, nor the different 

 sizes of the specimens. The proportion of length, width and thickness is as 

 variable as the sizes of the different shells. The illustrations on jjlate 2 give 

 the sizes of average specimens. 



Formation and Locality. Found associated with Ptyohodesma knappiana in the oherty layers 

 of the Devonian limestone, in Jefferson county, Ky., and in Clark county, Ind., at and around the Palls of 

 the Ohio. It occurs rather abundantly in fractional shells, but fair specimens are not often met with. 

 The shell illustrated by figure 14, plate 2, has two beautiful orania attached to its surface. 



