:>5G BRITISH SILURIAN BRACHIOPODA. 



under the two curved brachial lamellae, maybe seen a strong median ridge, which extends 

 about two thirds of the length of the valve, while on either side the large concave 

 adductor muscular impressions occupy a considerable space. 

 Length G, breadth 7 lines. 

 Obs. The shape and size of the muscular scars, as well as the strong prominent 

 median ridge, seem to distinguish this species from its Silurian congeners. It was 

 discovered by Mr. H. Wyatt-Edgell in the Caradoc Sandstone at Horderley, in Salop. The 

 description above given is taken from gutta-percha moulds made from internal casts : the 

 original specimen is in the Museum of Practical Geology. 



Orthis crispa, M'Coy. PL XXXVIII, figs. 5—10. 



Orthis ckispa, M'Coy. Synopsis Sil. Foss. Ireland, p. 29, pi. hi, fig. 10, 1846 ; and Brit. 

 Pal. Foss., p. 216, pi. i h, fig. 43, 1852. 

 — — Salter. Siluria, 4th ed., p. 526, 1867. 



Spec. Char. Subquadrate, or nearly semicircular, rounded or slightly indented in front ; 

 hinge-line as long as, or slightly less than th& width of the shell. Ventral valve gently 

 convex, with a slight mesial depression commencing at about the middle of the shell, and 

 widening as it nears the front ; area narrow ; fissure triangular, open. Dorsal valve 

 evenly convex, and deeper than the opposite oncj hinge-area very narrow. Surface 

 of both valves covered with numerous radiating, bifurcating, obtusely angular ribs, 

 crossed by numerous equidistant, close, prominent, concentric, scale-like lines of growth. 

 Length 9, width 10, depth 4 (or more) lines. 



Obs. This species is remarkable on account of its squarish shape, as well as from the 

 numerous concentric scale-like hues which intercept the radiating ribs, and which impart 

 to the surface some resemblance to Orthis Bouchardii. The number of radiating ribs 

 varies in different specimens, from forty-five to fifty having been counted round the 

 margin by Prof. M'Coy ; but I cannot agree with him when he states " it is possible that 

 0. scabrosa, Davidson, may be the adult of this species," both forms being perfectly 

 distinct generically, as well as by their external shape and internal character, &c. 

 The interior of 0. crispa presents the usual characters of the genus Orthis. 



Position and Locality. This shell occurs chiefly in the Caradoc, but has also been 

 found by I\Ir. Hughes in the Lower Llandovery at a quarter of a mile south-east of 

 Cwmrhyddan, in Wales. In the Caradoc it is not rare at Bettws-y-Coed, in Caernarvonshire ; 

 at Llanfyllin, and Meifod near Welshpool, in Montgomeryshire ; and in Bala Limestone at 

 Helms Gill, Dent, Kendal. In Ireland it occurs, according to Prof. M'Coy, at Bardahes- 

 siagh, Pomeroy, County Tyrone ; but is especially abundant in the schists of Knock- 

 mahon, Tramore, County Waterford, &c. 



