186 BRITISH SILURIAN BRACHIOPODA. 
by about twenty-four angular bifurcating ribs, which are crossed by concentric lines 
of growth. 
Length 8, width 13 lines. 
Obs. Of this species I have never been able to examine more than two imperfect 
specimens, those figured by J. de C. Sowerby in the ‘Silurian System, and preserved 
in the Museum of the Geological Society of London. One of these is the internal cast of 
a ventral valve, while the other is a cast or impression of the exterior of the same valve; 
and on such scanty and incomplete material it is impossible to completely describe the 
species, or to say much with reference to its specific value. It differs, however, sufficiently 
in shape from other British species of the genus with which we are acquainted, and 
especially so from those that occur in the Llandovery rocks, as to lead us to retain it, at 
least provisionally, as a distinct species. 
Position and Locality. Lower Llandovery beds; Goleugoed, Llandovery. 
RHYNCHONELLA T'HoMsoNI, sp. nov. Pl. XXIV, fig. 18. 
TEREBRATULA, sp., Salter. Quarterly Journal Geol. Soc., vol. vii, p. 117, pl. vii, 
fig. 3, 1851. 
Spee. Char. Shell small, somewhat trigonal and compressed, longer than wide, 
broadest anteriorly, tapering at the beaks; front straight. Valves moderately and 
uniformly convex ; smooth to within a short distance from the margin, where in the dorsal 
valve, close to the front, there exists a wide flattened or slightly concave mesial fold of 
small elevation. One or two short ribs occur also on the lateral portions of the shell. In 
the ventral valve one or two short ribs are also present near the front. Sinus wide, short, 
and very shallow. Beak small, moderately incurved, with a small aperture under its 
angular extremity. : 
Length 5, width 4, depth 2 lines. 
Oés. In 1851 this small shell was well figured by Mr. Salter, but without description 
or specific designation. When quite young no fold, sinus, or ribs, are observable, while in 
more aged examples they exist only close to the frontal margin. I have named it in honour 
of Mr. J. Thomson, of Glasgow, to whom we are indebted for the discovery of several 
important Ayrshire Silurian species. 
Position and Locality. Rh. Thomsoni is not exceedingly rare in the Caradoc Limestone 
at Craig’s Head Quarry, near Girvan, in Ayrshire. It is said to have been found also at 
Penwhapple Glen. 
