186 SUPPLEMENT TO THE 



Museum from the Middle Caradoc of Pen-y-Gaer Druidion, Haford Evan, Milltefe 

 Carrig ; Pentre Cwmdu, Brolch Llandeillo ; west side of Garn Bryn ; Tan-y-Bulch-y-groes, 

 Llandrillo ; Alt-yr-Anker, Meifod Cefn-y-coedog, Llangedwgn, and several other places. 



In dimensions, none of the specimens of OrtJds retrorsistria seem to have quite 

 attained 9 lines in length by 11 in breadth; and it is consequently a smaller shell 

 than Orthis alternata. Sow., which attained, and at times exceeded 10 lines in 

 length by 14 in breadth, the measurement of Sowerby's type specimens. There exists 

 also a difference in the shape and dimensions of the muscular impressions in the interior 

 of the valves of Ortliis alternata, and it was because Mr. Salter had confused the two 

 species that, in following him, I said, at p. 266 of my ' Silurian Monograph,' " that the 

 internal details of OrtUs retrorsistria are so like those in 0. alternata that they need not 

 be repeated." The shell is also, as seen in specimens from Horderley, in the Wood- 

 wardian Museum, flatter, and the ribs more numerous, or somewhat more finely marked 

 than in M'Coy's 0. retrorsistria^ and therefore, as above stated, I am disposed to regard 

 0. alternata and M'Coy's species as distinct. 



OrtJds retrorsistria is marginally semi-circular and generally a little broader than 

 long, the hinge-line slightly shorter than the greatest breadth of shell, the ventral valve 

 moderately convex, the area not exceeding a line in breadth at the widest part. Dorsal 

 valve almost flat, with a very slight median depression, not observable, however, in every 

 specimen, hinge-area narrow ; surface marked with very numerous radiating, thread-like, 

 raised striae, with one or two or three smaller or shorter ones between the larger pair, 

 and assuming a divaricating curve at the sides tovi^ards the cardinal margin. In the 

 interior of the ventral valve the muscular area is large and in the shape of two very- 

 elongated, sub-parallel, slightly deviating spaces margined by a narrow ridge, and 

 divided from one another by a free space of somewhat lesser breadth. In the interior 

 of the dorsal valve a small cardinal process is situated between two prominent processes, 

 beyond which are small, deep pits for the reception of the teeth of the opposite 

 valve, these lie close to the narrow hinge-area of the valve. Under the cardinal process 

 commences a very prominent widish ridge which extends to upwards of four or five fifths 

 of the length of the valve, which curving round forms a prominent convex ridge 

 following closely, but at a little distance from the margin of the shell. On each side of 

 this median ridge are seen the muscular depressions and which occupy a little less than a 

 third of the breadth of the shell as well as of its length. The vascular impressions are 

 plainly seen, also becoming often bifurcated and especially so near the margin of the valves. 



As we have already observed, the shell or the impressions left by it are very often out 

 of shape and distorted to one side or to the other. In PI. XXXVI, figs. 39 — 42, I have 

 represented M'Coy's original figures, and in PL XIV of this Supplement I have given a 

 series of drawings carefully selected from the best and most typical specimens of the 

 species in the Woodwardian Museum. 



I 



