260 BRITISH SILURIAN BRACHIOPODA. 



Orthis sagittifera, M'Coy. PI. XXXVI, figs. 18—23. 



Obthis sagittifera, M'Coy. Ann. Nat. Hist., 2nd ser., vol. viii, p. 398, 1851 ; and 

 Brit. Pal. Foss., p. 227, pi. in, figs. 1.5—19, 1852. 

 — — Salter. Mem. Geol. Survey, vol. iii, p. 268, 1866; and Siluria, 



4th edit., p. 527, 1867. 



Spec. Char. Rotundato-quadrate or semicircular, slightly wider than long, depressed ; 

 hinge-line as long as, or a little less than the shell's width ; cardinal extremities obtuse ; 

 lateral margins almost parallel, front rounded. Ventral valve gently convex, and slightly 

 longitudinally carinated along the middle ; beak small, incurved, not projecting ; area 

 triangular, narrow ; fissure wide, open. Dorsal valve moderately convex, deeper than the 

 opposite one, with a narrow mesial sulcus extending from the extremity of the umbonal beak 

 to about the middle of the shell ; hinge-area very narrow. Surface of both valves radiated 

 with very numerous fine, thread-like, bifurcated, raised striae, no fasciculation. In the 

 interior of the ventral valve is a prominent hinge-tooth on each side of fissure, supported 

 by dental plates, which partly enclose a saucer-shaped muscular depression with raised 

 margin, and divided longitudinally by a widish median ridge. In the interior of the 

 • dorsal valve a small cardinal process is situated between two diverging brachial processes, 

 under which a longitudinal ridge extends to about half the length of valve, dividing into 

 two pairs the quadruple adductor impression, each pair being likewise separated by a 

 small oblique ridge. 



Length 14, width 15, depth 4 lines. 

 Obs. The internal cast of the dorsal valve (fig. 21) is often recognisable by its arrow- 

 shaped slits, which are due to the median ridge and brachial processes. Prof. 

 M'Coy observes that " this species is, perhaps, most nearly allied to 0. turgida, M'Coy, 

 from which it is distinguished externally by its flatter ventral valve, with nearly straight 

 profile, and a cardinal area less than half the height, and a much more obtuse apical 

 angle. The striae are thicker and coarser. Internally the difference is still greater, the 

 present species having much shorter and wider muscular impressions in the ventral valve, 

 and the characteristic broad-arrow-shaped impressions of the rostral portion of the dorsal 

 valve, produced by the comparatively long, divergent, dental lamellae [brachial processes], 

 wholly separated from the mesial furrow. The gibbous dorsal valve, with its deep, 

 narrow, mesial sulcus near the beak, coarser striae, and totally different internal characters, 

 separate it from the 0. retrosistria, M'Coy, with which it also frequently occurs." 



Position and Locality. It occurs in the Caradoc and Lower Llandovery, and is 

 extremely abundant in the decomposing sandy Bala Schists of Aber Hirnant, east of 

 Bala, Merionethshire. A fine series of specimens may be seen in the Cambridge 

 Woodwardian Museum, collected by our veteran and celebrated geologist the Rev. A. 



