ORTHIDiE. 265 



true Orthis. The very large ventral muscles are peculiar and characteristic. There 

 is altogether a curious imitative appearance about this shell. Fragments have been 

 constantly mistaken for Sfrophomena expansa or Leptcena sericea. To both genera it has 

 relation." In this last view I quite concur, but must, for the reasons above given, retain 

 it as an Orthis. Mr. Salter considered OrtUs retrorsistria, M'Coy, to be a synonym of 

 0. alternata ; and although he may be correct in this view, the material in my possession 

 is not sufficient to enable me to corroborate his statement. 



Position and Locality. It appears to be common in Caradoc slate rocks throughout 

 Wales and Shropshire. Murchison gives us the following localities : — Whittingslow, 

 Soudley, and east flank of the Caradoc, Alt-yr- Anker, and the Maen Meifod, Lower Lickey 

 Ridge, east flank of Berwyns, Mandinam Llandovery. In the Museum of the Geological 

 Survey are good specimens from Ketch and Pentre Lymru, Llanfyllin, Cheney Lougville, 

 Penmachno, Caernarvonshire ; south-east of Cerrig-y-Druidion, Glyn Ceiriog, Pentre-cwm- 

 dda, and south of Glyn Diffwys in Denbighshire. At Llanfyllin, Meifod, Llanwddyn, north 

 of Llangedwyn, and Llangynnog in Montgomeryshire ; east and south-east of Bala Lake, 

 and Pont-y-Glyn-Diff"wys in Merionethshire ; &c. 



Var. ri:trorsistria, M'Cot/. PI. XXXVI, figs. 39—42. 



Orthis retrorsistria, M'Coy. Brit. Pal. Foss., p. 224, pi. in, figs. 12, 13, 1852. 



This is considered by Mr. Salter to be a small variety of Sowerby's Orthis alternata ; 

 and as I have not had the opportunity of examining many specimens of the shell, I here 

 reproduce Prof. M'Coy's figures and partly his description : — " Rotundato-quadrate, 

 depressed, no mesial ridge or furrow in either valve ; hinge-line nearly or quite as wide as 

 the shell ; cardinal angles slightly obtuse ; cardinal area flat, triangular, six or seven times 

 wider than high in ventral valve, only one third of this height in the dorsal valve. 

 Ventral valve gently convex, greatest depth about the middle of the length. Dorsal valve 

 flat round the margin, gently concave in the middle ; both valves vi^ith a few concentric 

 wrinkles of growth, about a line apart, and radiated with slightly irregular obtuse striae, 

 which branch into two or three at two or three intervals between the beak and margin, so 

 that each of the strong primary ones forms from seven to ten at the margin, separated by 

 a rather deeper sulcus from the adjoining ones, so as to produce a flat, indistinctly marked 

 fasciculation ; intervening sulci about the same size as the striae (obscm-ely punctured in 

 some specimens), which are straight in the middle, gradually assuming a divaricating 

 curve on the sides, which is so great near the angles that a large number of the lateral 

 striae curve backward from the beaks to terminate along the distal half of each side of 

 the hinge-line, instead of at the lateral margin ; all the striae are crossed by indistinct 

 transverse lines of growth. The size of the striae does not vary much in the various parts 



34 



