FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 53 



Fentamerus nucleus. HALL. 



Plate XXVII., figures 25 26 and 27, and Plate XXXIII., figures 27, 28, 29, 31, 32 and 33. 

 Pentamerus nucleus, Hall. 24th Rep. N. Y. St. Mus., p. 200a 1872. 

 Pentamerus nucleus, Hall. 27th Rep. N. Y. St. Mus., pi. 9., figs. 30 to 321875. 



Shell of medium size, sub-globose ; in the typical specimens width and 

 length about equal, but in the shells represented by figures 81, 32 and 33, 

 on plate 83, the length exceeds the width considerably. Ventral valve con- 

 siderably larger than the dorsal, and very ventricose ; cardinal margins 

 strongly rounded, forming, in most specimens, a regular curve, which joins 

 the somewhat stronger curve of the lateral margins. In some shells, however, 

 the cardinal margin is almost straight, equalling the greatest width of the 

 shell, and making the cardinal extremities either only slightly rounded or 

 obtuse angular, as may be seen in fig 31, plate 33. The umbo is very promi- 

 nent, and the beak strongly incurved over a small triangular foramen. The 

 middle of the valve, from below the umbo to the front, is slightly elevated, 

 forming a small mesial fold, and which consists usually of two, but sometimes 

 of three elevated plications. Dorsal valve transversely elliptical, or sub-oval, 

 only moderately convex ; umbo somewhat inflated. The middle of the valve, 

 below the umbo, depressed, forming a mesial sinus, which contains one or two 

 plications ; the beak is small and strongly incurved into the opposite valve. 

 Surface is marked by angular plications, usually two on each side of the mesial 

 fold or sinus ; these ribs do not reach very far back, never, or very seldom, ex- 

 tending beyond the middle of the valve, and in most specimens only noticeable 

 at and near the margin of the base. This species resembles somewhat Pent, 

 galeatus of the Lower Helderberg group, especially in those forms as repre- 

 sented by figure 31, plate 33, but it differs, in its shape and in the arrangement 

 of its plications, sufficiently from that species to be distinguished from it at the 

 first glance. It occupies an intermediate position between Pent, galeatus of 

 the Lower Helderberg, and Pent, fornicatus of the Clinton group. In regard to 

 the shells figured on plate 33, figures 31, 32 and 33, and even those figured on 

 same plate, figures 27, 28 and 29, I was uncertain for some time whether to 

 place them with this species or to refer them to Pent, galeatus. Since then I 

 found specimens of intermediate forms, uniting said shells with the typical 

 Pent, nucleus. 



Formation and Locality. These shells are found in the Niagara rocks of the quarries east of the 

 city of Louisville. Several good specimens were found by different collectors. 



