RHYNCHONELLID^. 



149 



species is characteristic of the Lower Pentamerus-Hmestone (Lower Ludlow rock) of 

 the Lower Helderberg group ; and large masses of the rock are often made up of the 

 broken and separated valves of this fossil. The more perfect specimens are obtained 

 from the shaly limestone above the Pentamerus-limestone, Helderberg Mountains, 

 Schoharie, Carlisle, Cherr\wally; Herkimer Co., Catshill, Hudson, and numerous other 

 places in New York State ; Cumberland, in Maryland, Decatur Co., in Tennessee, and 

 at numerous other points." Prof. Schnur describes and figures Devonian specimens from 

 the Eifel, which exactly resemble those from our Silurian deposits. 



Pentamerus linguifer, /. lie C. Sow. (sp.). PI. XVn, figs. 11 — 14. 



Atrypa lixguifera, J. de C. Sow. Sil. Syst., pi. xiii, fig. 8, 1839. 

 Pkntamerus linguifer, Davidson and De Verneuil. Bull. Soc. Geol. France, 2m\ 

 ser., vol. V, pp. 333 and 346, 1848. 



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



p. 277, 1848. 



— Bubo, Barrande. Sil. Brach. Bohmen. Naturvr. Abhandl., vol. ii, p. 116, 



1847. 



— LINGUIFER, D'OrS, Prodrome, vol. i, p. 41, 1849. 



— — Morris. Cat. Brit. Foss., p. 142, 1854. 



— — F. Schmidt. Sil. Form. Ehstland, &c. ; Archiv Naturk., 



&c., vol. ii, p. 212, 1858. 



— — Salter. Siluria, 2nd edit., pi. xxii, fig. 21, 1859. 



■ — ROTUNDUS, Lindstrom. Proc. Royal Acad. Stockholm, p. 365, pi. xii, 

 fig. 6, 1860. 



Spec. Char. Sub-globose or orbicular, slightly wider than long, very rarely longer 

 than wide ; ventral valve gibbous, becoming very ventricose in old specimens, and espe- 

 cially near the beak, and divided longitudinally along the middle by a narrow groove, as 

 well as by a broad shallow sinus ; beak moderately large and somewhat compressed along 

 the middle, greatly incurved, so as almost to come into contact with the umbone of the 

 dorsal valve, the small triangular fissure beneath being rarely exposed. Dorsal valve 

 somewhat transversely oval, and divided into three portions by a moderately elevated 

 mesial fold ; front raised into rounded waves ; surface smooth, marked only by a few 

 concentric lines of growth. In the interior of the ventral valve, the dental lamellae are 

 developed into a small V-shaped cavity or chamber, extending freely into the cavity of 

 the shell, and supported by a small vertical septum only at its posterior extremity. Inside 

 the dorsal valve two very narrow septa, with a small intervening space, support two small 

 plates, curving outward, and corresponding with those of the opposite valve. Two 

 specimens measured — 



Length 9, width 10 depth 7^ lines. 

 » 10, „ 9, „ 7i „ 



