FOSSILS OP THE SILURIAN AND DEVONIAN ROCKS. 209 



more or less strong, concentric folds or elevations, which are angular on the 

 anterior part of the shell, and become rounded and lower posteriorly. They 

 increase in size from the beak to the margins. In size this shell varies greatly ; 

 the forms illustrated give the average dimensions of this species. 



Professors Hall and Whitlield consider it so closely allied to Grr. secunda, 

 as to make a variety of that species ; but to me, the differences between this 

 shell and the mentioned species appear so pronounced, that I place it in an en- 

 tirely different species, retaining Hall's name, "gibbosa." 



Formation and Locality. Occurs in the cherty layers topping the hydraulic limestone of the 

 Devonian formation, in Kentucky and Indiana, at and around the Falls of the Ohio. 



Genus Paracyclas. Han. 



Paracyelns, Hall. Geol. Surv. of N. Y., Kep. of 4th Dist. 1843. 

 Etymology: para, allied to; cyclas, a genus of shells. 

 Type: Paracyclas elliptica. 



Shell equivalve, sub-equilateral, sub-orbicular or broadly sub-elliptical. An- 

 terior end regularly rounded ; posterior end rounded or sub-truncate, some- 

 what more produced below than the anterior ; beaks small and low, generally 

 rising little above the hinge- line. 



Hinge-line short ; post-cardinal slope more or less defined by an oblique 

 furrow or depression, which sometimes leaves the extremity sub-alate. 



Surface concentrically striated; sometimes with strong concentric ridges 

 marking the exterior. 



Structure of hinge not fully observed. Ligament supported on each side, 

 internally, by a narrow plate, and leaving in the cast two diverging grooves 

 directed forward from the beak. Muscular impression on the post-umbonal 

 slope. Pallial line parallel with, and a little within, the margin of the shell. 



This fossil possesses many of the external characters of the finely striated 

 forms of modern Lucina ; and the distinguishing characters are not strongly 

 marked. 



(Copied from Hall's Lamellibranchiata. ) 



Paracyclas elliptica. HALL. 



Plate II., figures 1, 2 and 3. 



Paracyclas elliptica, Hall. Palaeont. N. Y., Vol. 5, part 1, page 440, plate 72, figures 23 to 33. 

 Lucina elliptica, var. occidentalis, H. and W. 24th Ann. Rep. N. Y. St. Mus., page 189. 



Shell large, sub-circular or broadly elliptical ; length and height about equal. 

 Pallial margin regularly curving from the extremities of the hinge. Cardinal 

 line short, more than one-third the length of shell, slightly arcuate. 



GEOL. SUR. 27 



