ORTHIDtE. 257 



Orthis unguis, Soic. (sp.). PI. XXXVII, figs. 16—22. 



Terebeatula unguis, J. de C. Sow. Sil. Syst., pi. xxi, fig. 13, 1839. 

 Oethis — Salter. Siluria, 4th ed., p. 527, pi. v, figs. 3, 4, 1867. 



Spec. Char. Orbicular, as wide as long, or slightly transverse, broadest across the 

 middle ; lateral and frontal margin regularly rounded ; hinge-line much shorter than the 

 breadth of the shell. Ventral valve convex, slightly keeled along the middle; area 

 triangular, about one fourth as high as long ; fissure open ; beak incurved, but scarcely 

 projecting. Dorsal valve very slightly convex, or almost flat, longitudinally depressed 

 along the middle ; hinge-area very narrow. Surface of valves ornamented with from 

 sixteen to about twenty-four angular ribs, which in some specimens bifurcate as they near 

 the margin. 



Length 6, breadth 6^, depth 4 lines. 



Ohs. The internal characters are those usual to the genus Orthis, and present nothing 

 particular. It seems strange that Mr. J. de C. Sowerby should have mistaken it for a 

 Terebratula. It is not a large shell, rarely exceeding five and a half fines in length, and 

 usually occurs under the condition of internal casts and external impressions. In some 

 examples all the ribs are simple, but in others they bifurcate near the margin, as may be 

 seen in figs. 17 and 19 of our plate. 



Position and Locality. It occurs in the Caradoc Sandstone at Horderley, Cheney 

 Longville, and Gretton, in Shropshire. In Scotland it has been found in Limestone at 

 Craighead Quarry, near Girvan, in Ayrshire. 



Orthis protensa. Sow. PI. XXXVT, figs. 24—30. 



Orthis protensa and 0. lata, J. de C. Sow. Silurian System, pi. xxii, figs. 8 — 10, 



1839. 



— lata, Phillips and Salter. Mem. Geol. Survey, vol. ii, p. 289, 1848. 



— PROTENSA, IPCoy. British Pal, Foss., p. 224, 1852. 



— LATA, Morris. Cat., p. 140, 1854. 



— LATA, var. PROTENSA, Salter. Mem. Geol. Survey, vol. iii, .pp. 276 and 361, 



1866. 



— PROTENSA (=0. lata), Salter. Siluria, 4th ed., p. 526, pi. ix, figs, 22, 23, 



1867. 



Spec. Char. Transversely semicircular or semicylindrical, depressed, wider than long, 

 very slightly rounded or nearly straight in front ; hinge-line almost or quite as wide as 

 the shell. Ventral valve very gently convex, depressed in the middle and towards the front ; 

 area narrow, fissure open. Dorsal valve rather more convex than the ventral valve, area 



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