ORTHIDA. 233 
Surface of valves ornamented with from eighteen to thirty simple rounded ribs, with deep 
concave interspaces between each pair of ribs of equal or slightly greater breadth, the 
whole surface being likewise marked or finely imbricated by minute, radiating, longitudinal 
strize, crossed by very fine, equidistant, concentric lines. Interior not known. A large 
specimen measured— 
Length 6, width 10 lines. 
Obs. ‘This species was originally described by Mr. J. de C. Sowerby as a Sprrdfer ; 
but its true place in the genus Orthis was subsequently assigned to it by Mr. Salter. It 
varies very much in shape, on account of a greater or lesser prolongation of its cardinal 
extremities or “ expanded cuspidate sides,” as described by Mr. Sowerby. The specimens 
figured in the ‘ Silurian System’ are also somewhat out of shape, the shell not being usually 
quite as transverse as therein represented. 
Position and Locality. Orthis alata occurs low down in the Llandeilo formation at 
Perkins Beech and Lords Hill, Shelve, where it is tolerably abundant in the condition 
of external casts and impressions, the shell itself not having been obtained. Sowerby’s 
examples are stated to have been found at Pensarn and Mount Pleasant, Caermarthen, 
also in the Lower Llandeilo, to which stage the shell seems to be apparently restricted, at 
least so far as we are at present aware. 
Orvais Berruotsi, Rovauil (7), var. pRRATICA, Dav. Pl. XXXII, figs. 21—28. 
? Orruis Berruorst, Rowault. Bull. Soc. Géol. de France, 2nd ser., vol. vi, p. 68, 
pl. ii, figs. 4, 4a, 6, c, 1848. 
> — Sharpe. Quarterly Journal Geol. Soc., vol. ix, p. 154, pl. viii, 
fig. 4, 1853. 
Spec. Char. Shell ovate, or slightly transversely oval, rather wider than long, broad- 
est about the middle; hinge-line short, not more than half, and often less than half, the 
width of shell. Ventral valve very moderately convex, with or without a slight mesial 
depression, commencing at about the middle of the valve and widening as it nears the 
front. Area narrow and small. Dorsal valve slightly convex, with sometimes a shallow 
median depression, which, beginning at the umbo, dies out before reaching the middle 
of the shell; both valves are covered with numerous thread-like bifurcating striz, which 
divide several times before reaching the margin ; the valves are likewise crossed at irregular 
intervals by strong concentric lines of growth. In the interior of the dorsal valve there 
is a large elongated, oval-shaped, prominent, cardinal process between two triangular 
brachial processes or plates, on the upper outer side of which, and close under the 
small hinge-area, are situated the sockets, while a thickened rounded ridge extends from 
under the cardinal process to about half the length of the bottom of the valve, and on 
either side of this are the concave adductor muscular oval-shaped impressions. (These 
30 
