LOWER HELDERBERG ROCKS. " J861 



This species presents very marked differences from the other species of tlie genus 

 known in our strata. The larger valve is the dorsal, and bears the two internal 

 septa; while the smaller valve, or that with the sinus, is the ventral valve, having 

 the triangular cavity beneath the beak, with a perforation at the extremity, and the 

 dental lamellae are produced into the elongated cavity represented in fig. 1 m, x ; 

 and which, from the thickening of the valve, is often affixed to the shell at its base 

 without the intervention of the usual septum, which, when present, is a very sub- 

 ordinate feature. The dental lamellae are lobed on the outside as shown in the 

 figures above cited, leaving a space for the interlocking of the septa or brachial 

 lamellae of the opposite valve. The dorsal valve is marked by two converging septa, 

 which extend scarcely more than one- third of the length of the shell, and terminate 

 in a thickened ridge in the deepest part of the valve. From these laminae or septa, 

 near their junction with the hinge line, originate two broad thin brachial plates, 

 which are very imperfectly shown in fig. 1 t. 



This beautiful species, in the reversed character of the valves, resembles one from 

 the Island of Anticosti, named by Mr. Billings P. reversus; but the latter is much 

 more coarsely plicated, and without the interstitial plications. The species now de- 

 scribed bears a very close resemblance to Pentamerus {^Atry-pa) interplicata of the 

 Niagara group; which, although its internal structure has not been ascertained, I 

 have no doubt is a true Pentamerus. The P. vemeuili, however, is readily distin- 

 guished from that one by its larger size, more globose form, and more numerous 

 plications. 



In the young shells of this species, the dorsal valve is abruptly incurved; the 

 beak of the ventral valve is nearly as high as the opposite, and has beneath it a 

 distinct triangular foramen and an apparent area, which gives the cardinal line the 

 appearance of that of an Orthis. As the shell grows older, the beak of the opposite 

 valve fills the foramen; and the notch in the beak is carried farther inwards as the 

 shell thickens, so that in old shells the beak presents a somewhat circular perfora- 

 tion, which communicates below with the triangular cavity. 



In extremely young shells, there is no appearance of mesial sinus or elevation. 



Fig. 1 a, 6. Ventral and front views of a young specimen. 

 Fig. 1 c, d. Dorsal and profile views of a specimen larger than the preceding- 

 Fig. 1 e. The cardinal view of fig. \ c, d enlarged, showing the triangular foramen and ap- 

 parent area. 



Fig. \ f. Dorsal valve of a finely plicated specimen. 



Fig. 1 h. The same enlarged, to show the regularly intercalated smaller plications. 



Fig. 1 g. Ventral view, showing the bifurcating plications. 



Fig. 1 i, k, I. Ventral, dorsal and profile views of a specimen with simple coarse plications. 



Fig. 1 m. Front view of a specimen similar to the preceding. 



