382 BRITISH PALALOZOIC FOSSILS. [ Bracutopopa. 
surface radiated with very numerous, simple, equal, flattened, or slightly convex ridges (five in two lines at eight 
lines from the beak), fifteen to twenty occupying the sinus, and about twenty-two on each side. Length of 
rather small specimen eleven lines, proportional width =, width of sinus 7%, depth =; texture of the shell 
densely fibrous. 
The margins are much sharper or more angular than in the Silurian 1. W7lsoni, and the ribs simple ; 
another difference is the small size at which the deflection of the margin begins in that species; there is no trace 
of it in the present one, which never acquires the great semicylindrical extension of the Silurian shell. 
Position and Locality —Common in the Devonian limestone of Plymouth and Torquay. 
HeEMITHYRIS PLEURODON (Phill. Sp.) 
Ref. and Syn. = Terebratula id. Phil. G. Y. Vol. II t. 12. f. 25 to 28. = Atrypa fallax Sow. Geol. Trans. 
2nd Series, Vol. V. t. 54. f. 15. 
Sp. Ch.—Transversely elliptical, or subpentagonal; entering valve very slightly arched from the beak to 
the front margin, which is much elevated into an oblong sinus, forming a flat mesial ridge, slightly marked at 
the margin ; sides very much arched downwards towards the front, slightly excavated at the posterior or lateral 
margins; receiving valve with a narrow, prominent, slightly incurved beak, with a large triangular opening 
beneath it; sides nearly flattened ; middle portion, forming the wide flattened sinus, semicircularly arched from 
the beak to the front margin; neither the mesial ridge nor sinus marked in the rostral portion; four to six large 
angular plaits, separated by very deep re-entering angles, raised with the front margin, with on each side five 
sharply angular much arched ribs, slightly larger than the mesial ones, and forming very deep angular indenta- 
tions in the margins; behind these nearer the beak are usually two or three smaller ones, all the ridges continued 
sharply defined up to the beak ; shell densely fibrous under the lens. Width ten lines, proportional length S, 
depth of both valves 7. 
In this species the two dental lamelle diverge at about 45°, and reach one-third the length of the shell in 
the receiving valve; the mesial septum in the entering valve seems somewhat longer. There is not the slightest 
difference in any respect between the varieties found in the slates and subordinate limestone of S. Petherwin, 
called Atrypa fallax by Sowerby, and those in the carboniferous limestone of Derbyshire. Mr Fischer’s 
specimen of his 7’. pentatoma is so very defective at the sides, that neither the number nor character of the lateral 
ribs can be recognised, and I therefore prefer retaining Mr Phillips’s specific name, the type for which is perfectly 
clear. 
Position and Locality—Common in the slates and subordinate limestone of S. Petherwin;? in the green 
chloritic slate of Tintagel; slate in road-quarry, half a mile S. of Marwood. 
HEMITHYRIS PROTRACTA (Som. Sp.) 
Ref. = Atrypa protracta Sow. Geol. Trans. 2nd Series, Vol. V. t. 56. f. 16. 
Sp. Ch.—Elongate, obtusely triangular; apical angle 80°; greatest width near the front ; posterior lateral 
margins long, front wide, obtusely rounded or subtruncate; the front margin abruptly raised into a large 
rounded sinus, only affecting the form of the valves near the margin, four large obtusely angular plaits 
raised with the front margin, but extending a very short way towards the beak; lateral margins minutely and 
obscurely crenulated ; valves moderately and equally convex ; receiving valve with a prominent, slightly ineurved 
beak, from which an obscure tumid ridge extends to the mesial plaits of the sinus; entering valve moderately 
gibbous along the middle, sloping to the sides; surface smooth. Length (of small specimen) five lines, 
proportional length of entering valve “%, width %, depth of both valves =. 
Position and Locality—Rare in the Devonian limestone of Plymouth. 
