ORTHID^. 255 



Orthis simplex, WCoy. PI. XXXII, figs. 10, 11. 



Orthis simplex, M'Coy. Synopsis of the Sil. Foss. of Ireland, p. 34, pi. iii, fig. 18, 

 1846. 

 — CALLIGRAMMA, var. SIMPLEX, Salter. Mem. Geol. Survey, vol. iii, p. 336, 



1866. 



Spec. Char. Shell rather small ; ventral valve subconical, about as wide as 

 long; hinge-line as long as, or slightly less than the vridth of the shell; area large, 

 divided in the middle by a triangular fissure. Dorsal valve nearly semicircular, and 

 slightly depressed along the middle; area narrow. Surface of valves regularly covered 

 by simple radiating, obtusely angular ribs. In the interior of the ventral valve the saucer- 

 shaped muscular cavity is longitudinally divided by a narrow ridge, which extends to 

 some distance along the bottom of the valve and beyond the margin of muscular cavity. 

 In the interior of the dorsal valve the cardinal process is rather large, while the brachial 

 process is short ; a narrow median ridge commences under the cardinal process, and 

 extends to more than half the length of the shell, while shorter, similar, and parallel 

 ridges margin the outer edge of muscular scars. 

 Length 5|-, width 5 lines. 



Obs. Of this species (?) I have seen only very few valves, and consequently cannot 

 say I know it well. Mr. Salter writes, " This, which is a very convex shell, is wider 

 than any of Pander's varieties, though most like his var, rotundata. It is a small shell 

 too, and has about thirty-two (M'Coy says twenty-four) equal ribs. ... 0. simplex 

 has well-defined longitudinal and cross striae, rather thicker hinge-teeth than usual, and 

 the space for the muscular impressions in the ventral valve is narrower than in the 

 majority of varieties." He considers it a variety of 0. calligramma ; but if the internal 

 details which I have figured are constant it must, I should think, be specifically distinct 

 from Dalman's species. 



Position and Locality. 0. simplex is a Caradoc species, and is stated by M'Coy to be 

 rare in the grits of Carrickadaggan, New Cross, County Wexford ; very common in the 

 soft decomposing schists at Knockmahon, Tramore, County Waterford, in Ireland. 



Orthis Salteri, Dav. PI. XXXVI, figs. 31—34. 



Spec. Char. Of this species the internal cast only has been hitherto discovered, so 

 that we can off'er but a very incomplete description of its characters. In shape it is 

 semicircular, and slightly wider than long. Ventral valve moderately convex ; hinge- 

 line shorter than the width of the shell ; area triangular, rather broad ; in the interior 

 the saucer-shaped muscular depression is small. In the interior of the dorsal valve, 



