152 



BRITISH SILURIAN BRACHIOPODA. 



Pentamerus oblongus, M'Cotj. Brit. Pal. Foss., p. 211, 1852. 



— L^vis, Id. Ibid., p. 209, 1852. 



— — (P. OBLONGUS, Sow.), Morris. Catal. Brit. Foss., p. 142, 1854. 



— OBLONGUS, Bigshrj. Quart. Journ. Geol. Soc, vol. xiv, p. 410, 1858. 



— — Salter. Siluria, pp. 100 and 230, fig. 2, and pi. viii, figs. 1, 



2, 3, 1859. 



— — Eichwald. Lethsea Rossica, vol. i, p. 788, 1859. 



— ESTHONUS, Lindstrdm. Proc. Roy. Akad. Stockholm, p. 365, 1860. 



— L^vis (young of P. oblongus), Salter. Mem. Geol. Survey, vol. iii, 



pp. 276 and 360, 1866. 



Spec. Char. Longitudinally oval or ovate, sub-trigonal ; valves moderately and almost 

 equally convex, in some cases depressed ; young shells evenly convex and smooth, older 

 shells usually much trilobed. Mesial fold in dorsal valve tolerably broad, of small elevation, 

 slightly depressed along the middle, while that on the ventral valve is rounded ; beak 

 moderately produced, and so much incurved as to touch the umbone of the dorsal valve; 

 fissure under the beak triangular, not much exposed ; beak-ridges sharply defined, and 

 leaving between them and the hinge-line a concave space (not a true area). Surface of valves 

 smooth, with sometimes a few irregular furrows and concentric lines of growth. Inside 

 the ventral valve a long vertical median septum, extending to two thirds or more of the 

 length of the valve, supports the conjoined trough-shaped dental plates. In the dorsal 

 valve two similar long, vertical septa, with a small free space between them, support two 

 outw^ardly sloping plates, which correspond with the trough-shaped plates or V-shaped 

 cavity of the opposite valve. Shell-structm^e impunctate. Two specimens measured — 

 Length 2 inches 7 lines, width 2 inches 3 lines, depth 1 inch 5 lines. 

 „ 8 „ 8 „ (P. IcBvis, Sow.) 



Ohs. Prof. J. Hall, at p. 79, vol. ii, of his 'Palaeontology of New York,' says, 

 " It is impossible to give a description which shall be applicable to every form of a 

 shell so variable as this species. In the smaller and medium-sized forms, in the 

 Pentamerus-limestone of the Chnton group at Rochester and elsewhere, the shell has a 

 general oval or ovate form, sometimes slightly trilobate at base ; it is so much depressed 

 that the thickness or depth of both valves is only about half the width. This proportion 

 sometimes continues even in very old shells, the trilobate character of the base being often 

 very consjoicuous. In the majority of specimens, however, the valves become gradually 

 more gibbous as the shells increase in size, and the trilobate character may be either 

 preserved or entirely lost. In young shells usually the surface is marked only by con- 

 centric striffi, though in some instances distinct hues of growth give a more uneven 

 surface, and this character becomes more prominent with increasing age. Although the 

 general and prevailing form is oval or ovate, yet we not infrequently meet with forms 

 that are rounded, and the ventral valve wider than long." What Prof. Hall has observed 

 with reference to the variations assumed by this shell in America will be found equally 

 api)licablc to those that occur in Great Britain and in Europe ; but none of the specimens 



