262 BRITISH SILURIAN BRACHIOPODA. 



so as Sfroph. Siluriana has a concave dorsal valve, and is consequently a true StropJion/ena 

 — certainly not an Orthis with a flat dorsal valve, as is stated to be the character of the 

 so termed Orthis Ilirmantensis. Some one or two small speciyiens of this last, sent 

 to me by Mr. Davies, and obtained by him in the Valley of the Hirnant, east of Bala, 

 in North Wales (fip;s. 8 and 9 of our plate), presented a very elongated aspect ; and 

 in an internal cast the vascular impressions were beautifully displayed, as will be likewise 

 seen by a figure in my plate. I should be very glad to be able to study some good 

 remains of the Hirnant shell, and shall feel obliged if any local geologist would kindly 

 afford me the opportunity. 



Position and Locality. This species occurs in the Caradoc or Bala division, and 

 is said by M'Coy to be very abundant in the oolitic limestone and decomposing schists 

 over the Bala Limestone at Aber Hirnant, east of Bala, North Wales ; and in a similar 

 limestone at Cwm-yr-Aethen ; oolitic limestone of Maes-y-fallen, Bala, Merionethshire ; 

 rare in the limestone of Cerrig-y-Druidion, Denbighshire. Prof. Harkness kindly sent me 

 the flat impression of a small dorsal valve he had obtained below the Graptolitic mudstones 

 of Westerdale, Sedberg. 



Orthis sarmentosa, M'Coy. PI. XXXVI, figs. 35 — 38. 



Orthis sarmentosa, M'Coy. Syn. Sil. Foss. Ireland, p. 34, pi. iii, fig. 17, 1846 ; and 

 Brit. Pal. Foss., p. 227, pi. in, figs. 25—28, 1852. 



Sj)ec. Char. " Transversely oblong, depressed ; hinge-line as wide as ,the shell, ears 

 rectangular; receiving [ventral] valve elevated towards the beak, which is prominent, 

 flattened towards the margin ; cardinal area large, triangular, inclining backward at 120°, 

 its height (in narrow specimens) one fourth of the width ; entering [dorsal] valve gently 

 convex along the middle, depth greatest at one third the length from the beak, shghtly 

 exceeding that of the ventral valve, with a triangular cardinal area about half the height 

 of that of the dorsal valve, but placed nearly in the plane of the lateral margins ; both 

 valves radiated with from fifty to sixty rather thick, flexuous, rounded ridges, frequently 

 and irregularly branching between the beak and the margin ; finer, closer, and less 

 branched towards the sides ; about five in two lines in middle of front margin. Internal 

 cast of receiving [ventral] valve with two very long, strongly marked, dental lamellge 

 diverging at 40°, nearly straight, with a very slight inward curve at their extremities, 

 forming the lateral boundaries of the very large triangular prominent pair of muscular 

 impressions, which reach rather more than two thirds the length of the shell, finely 

 sulcated longitudinally, and separated by a slender, indistinct impression of a mesial ridge. 

 Surface exterior to the muscular impressions strongly marked by the external ridges ; 

 internal cast of entering [dorsal] valve with a moderate elhptical pit of the simple rostral 



