ORTHID A. 221 
thread-like, radiating striz. In the interior of the dorsal valve a small cardinal process is 
situated between two deviating curved brachial processes; quadruple muscular scars 
feebly marked, and divided longitudinally by a narrow ridge. In the interior of the ventral 
valve the saucer-shaped muscular depression occupies about a sixth of the inner surface 
of the valve, while the centre is occupied by a rather large adductor muscular impression, 
divided by a small narrow ridge, the remaining portions of the depression being filled up 
by the divaricator muscular scars. Two specimens measured— 
Length 9, width 9 lines. 
EOP OR Ee Oy 
Obs. It was in 1846 that Mr. Salter named, described, and illustrated his O. reversa, 
and he tells us distinctly that his shell was round when young, more transverse when 
old, the dorsal valve uniformly convex, while a shallow depression is present near the front 
in the opposite one; and that his shell was described and figured from abundant internal 
and external casts in the sandstone of Galway, which specimens form part of Sir R. 
Griffith’s collection. He also adds that there are species much resembling this in similar 
strata in Ayrshire. Now, it must be observed that, with reference to these Ayrshire 
specimens, Mr. Salter did not positively identify them in 1846 with his Galway types, 
but simply observed that they much resemble them. Having obtained, through the kind 
medium of Sir R. Griffith, the loan of the original specimens described by Mr. Salter,’ and 
compared them with the Ayrshire specimens so named, I cannot help considering these 
last, if not a distinct species, at any rate as a well-marked variety, which I will conse- 
quently describe separately. I must also observe that Mr. Salter had, at the time, the 
circular form so much in view as the type of his species (figs. 11 and 12 of our plate), 
that in the explanation of his plate he distinctly intimates the transverse specimens to be 
a “supposed variety, deeply depressed in front.” 
Position and Locality. The Lower Llandovery (?) at Egool, Ballaghaderreen, County 
Mayo; Cappacorcogue, Cong, &c., County Galway, Ireland. Also Lower Llandovery, 
Mandinam, near Llandovery. 
Orrais reversa, Salt., var. Mutuockiensis, Dav. Pl. XXIX, figs. 14—18. 
Ortuts REVERSA, Salter. Quart. Journ. Geol. Soc., vol. vii, p. 171, pl. ix, fig. 13, 1857. 
— — M‘Coy. Brit. Pal. Foss., p. 225, 1852 (not of Synopsis Sil. Foss. 
Ireland). 
_— — Salter. Siluria, 2nd ed., p. 230, fig. 48, 3, 1859. 
Transversely oval, or slightly quadrate, broadest about the middle. Obtusely angular 
posteriorly, slightly indented anteriorly, rounded laterally. Hinge-line usually very short. 
1 These are drawn in our plate. 
