198 BRITISH SILURIAN BRACHIOPODA. 
be sought for, by which some of the internal generic and family characters might be 
determined. 
Tripresia Grayiz, sp.nov. Pl. XXIV, figs. 31—32; and Pl. XXV, figs. 9, 10, 11. 
Spec. Char. Subrhomboidal or obscurely lozenge-shaped ; longer than wide, greatest 
breadth lower than the middle of the shell or at the anterior half. Dorsal valve gibbous, 
divided into three lobes of almost equal width, the central one forming a prominent an- 
gular fold, which, rising gradually at a short distance from the extremity of the umbone, 
becomes most elevated shortly before reaching the front, while its sides slope rapidly in 
the manner of a roof. The lateral portions, or ears, close to the hinge-line are flattened, 
while the umbone remains convex. Ventral valve much less deep than the opposite one ; 
sinus deep, angular, extending from the extremity of the beak to the front, and defined 
by subangular margins. Beak nearly straight, much produced, usually large and taper- 
ing, but truncated at its extremity by a small circular aperture. Area triangular, some- 
times equilateral, but more often the length is somewhat less than that of the width at the 
hinge-line. Deltidium rather broad, and occupying about one third of the area space ; 
triangular, and most elevated along the middle. Surface of both valves marked by 
numerous thread-like, radiating striae, which are crossed here and there by concentric 
lines of growth. ‘Two specimens measured— 
Length 6, width 53, depth 5 lines. 
Obs. In external shape and size the dorsal valve somewhat resembles that of Atrypa 
eatans, Conrad and Hall, as represented in pl. xxxui, fig. la, 6, of the first vol. of the 
‘ Palzeontology of New York,’ but differs from it entirely by its fine thread-like, radiating 
striae, as wellas by the great extension of the area in its ventral valve. Having sent drawings 
of our shell to Prof. Hall, he remarked that it is generically identical with Zriplesia 
extans, but does not belong to the American species. The several Ayrshire specimens I 
have seen seem to denote that in the perfect shell there existed a small foramen at the 
extremity of the beak for the passage of a pedicle, but, owing to the difficulty of obtaining 
specimens in good condition, it is rarely seen. Its angular fold is also very remarkable, 
reminding us of that occurring in Rdynchonella acuta. Its combined characters make 
it easily distinguishable from all our other British Silurian Brachiopoda. 
I have named this shell in honour of Mrs. E. Gray, of Glasgow, to whom science is 
indebted for the discovery of several new species of Ayrshire Silurian fossils. 
Position and Locality. This species belongs to the Caradoc or Bala division. 
Several bivalve examples were found by Mrs. Gray in the limestone at Craig Head 
(Quarry, near Girvan, in Ayrshire, and dorsal valves had been collected by Mr. J. 
Thomson in a green brecchia in Penwhapple Glen, at about one hundred yards below 
