RHYNCHONELLID^. 



155 



Ruhde, Keskfer, Kattentach, Nudi, Jerwakant, Kerro, Fennern, Oberpliahlen, Rostla, &c. 

 In Sweden Dr. Lindstrom found it at Strora, Lilla Carlso, Klienteberg, Bogeklint. It 

 occurs also in the Upper Llandovery (?) in the neighbourhood of Cliristiania in Norway. 

 In North America in the Clinton group at Rochester and elsewhere ; at Springfield and 

 Drayton, Ohio ; on the west side of the Mississippi River, &c. M. de Verneuil 

 observes that in North America, as well as in England, P. oblongus occurs near the point 

 of junction of the Upper and Lower Silurian groups, but in those beds which appear .to 

 belong more properly to the Upper Silurian ; also that he found it at Richmond, Indiana, 

 Galena (Illinois), at Seulchoix, Green Bay; also at Limekiln Island, and other islands 

 of Lake Huron. Sir W. Logan has obtained it near the River Ottawa, and at Gaspe, 

 in Canada. Dr. Bigsby obtained it further west and north ; but it is not yet known, 

 nor the band containing it, in the Arctic basin. 



Pentamerus undatus, /. de C. Sow. (sp.). PI. XIX, figs. 4 — 9. 



Atrypa undata, J. de C. Sowerby. Sil. Syst., pi. xxi, fig. 2, 1839. 



— — Phillips and Salter. Mem. Geol. Surv., vol. ii, part 1, p. 279, 



1848. 



Pentamekus undatus, M'Coy. Brit. Pal. Fosg., p. 211, 1852. 

 _ _ Morris. Cat. Brit. Foss., p. 143, 1854. 



— — Salter. Siluria, 2nd edit, pp. 100 and 230, fig. 6 ; pi. viii, figs. 



5, 6, 7, 1859. 



— — Id. Mem. Geol. Survey, vol. iii, pp. 276 and 360, 186G. 



Spec. Char. Transversely oval ; much wider than long ; valves convex ; ventral valve 

 most so, even gibbous, with a wide, sharply defined sinus commencing at the extremity 

 of the beak and extending to the front, sometimes longitudinally divided along the middle 

 by a slight elevation ; beak much incurved. Dorsal valve with a wide rounded fold of 

 small elevation ; sides rounded, and slightly indented in front. Surface smooth, marked 

 only by concentric fines of growth. In the inside of the ventral valve the V-shaped 

 chamber formed by the union of the converging dental plates is small, as well as the 

 vertical septum which partly supports it. Two specimens measured — 

 Length 7, width 10, depth 6 lines. 

 „ 11, „ 16 lines. 



Obs. Of this fossil I have seen many internal casts and impressions, and never 

 the shell itself. In one of these (fig. 8^) the vascular markings are beautifully defined, 

 while in another (partly fractured) the outer surface of the converging dental plates, as 

 well as a small area, are exposed (fig. 8). Again, in the casts of one or two examples 

 may be seen a short longitudinal slit due to the vertical septum. Prof. M'Coy informs us 

 that "the internal plates are excessively small, but formed on the usual plan of 



