02 PALAEONTOLOGY OF KENTUCKY. 



shallow depression on both sides of the umbonal region extends from the car- 

 dinal line towards the front, where it connects with the mesial sinus ; the sinus 

 begins at the middle of the valve and forms a slight extension in front. 



In a few instances this extension bends abruptly upwards, and fits in a cor- 

 responding inflection in the dorsal valve ; beak moderate and closely incurved 

 upon the umbo of the other valve, showing a slight, rounded perforation in 

 the apex, which leads into the broad triangular fissure ; cardinal area scarcely 

 visible. Surface ornamented by strong, rounded or sub-angular plications, 

 which increase towards the margins by bifurcation as well as by interpolation, 

 and by lamellose, imbricating concentric striae or lines of growth, which often 

 give the shell a very rugose appearance, as may be seen in figure 1, plate 15. 

 The size of the specimens in this species differs greatly among the individuals 

 illustrated; the larger may be considered as of maximum size, while the 

 smaller are of average size. 



Formation and Locality. This species, together with Spirifera oweni, with which it is associated, 

 belong to the most common fossils of the Corniferous limestone, at and around the Falls of the Ohio in 

 Kentucky and Indiana. In the washes of the fields around Charlestown, Indiana, a collector can pick up, 

 after some hard rains, in a day's hunt, more than a hundred fair specimens of this species ; they are mostly 

 silicified, and often contain the spiral coils, as may be seen in figures 18, 20, 21 and 22. There is a pretty 

 little shell in the Niagara rocks, which is placed also in this species, though the difference in the exterior 

 features of the Devonian and Silurian shells is so conspicuous, that nobody can overlook it. For what 

 reason these two fossils are kept in the same species I can not understand, and I shall, therefore, separate 

 the Niagara specimens from the Devonian, at least as a variety. 



Atrypa reticularis, var, niagarensis, N. TAB. 



Plate XXXII., figures 5 to 8, and 44 to 47. 



Shell small, at least below medium size ; longitudinally sub-elliptical or sub- 

 ovate, plano-convex ; hinge-line less than the width of the shell ; cardinal 

 extremities rounded ; length somewhat greater than the width ; base regularly 

 curved, sometimes slightly inflected in the center ; lateral margins gently 

 curved. 



Ventral valve slightly convex in the umbonal region ; all around this and to 

 the base there exists a slight depression, which forms at the front the mesial 

 sinus; beak moderate and incurved upon the umbo of the other valve. In 

 some specimens the sinus is produced beyond the front, and the extension, 

 thus formed, often rapidly and abruptly bends upward, as may be seen in figure 

 7, plate 32 ; in others there is scarcely a sinus noticeable. Dorsal valve very 

 ventricose or gibbous, sloping abruptly from the middle, where the greatest 

 convexity is, toward the cardinal and lateral margins, but less abruptly towards 

 the front. No mesial elevation is indicated, except in those specimens where 

 the frontal extension of the ventral valve makes an abrupt upward turn ; here 



