130 DEVONIAN FAUNA. 
Localities.—This species is very abundant at Lummaton. More than one 
hundred specimens are in my Collection, four in the Torquay Museum, thirty-three 
in the Woodwardian Museum. From Wolborough there are sixteen specimens in 
Mr. Vicary’s Collection, two of which are larger than any figured by. Davidson, 
and eight in the Museum of Practical Geology. 
Remarks.—This is a peculiar, elegant, and distinctive form, though whether the 
two adjoining shapes Rh. impleca and Rh. angularis are really distinct species 
remains, as Davidson left it, a matter of doubt. It varies considerably in trans- 
verseness, being typically as long as wide, but occasionally slightly longer, and 
frequently, as described by Davidson, wider. Its fold also varies in prominence. 
The species is distinguished by its pentangular shape ; by the peculiar character 
of its various contours; by the low, rounded, close-set ribs, which vanish near the 
beaks ; by the vertical straightness of the centre of the dorsal valve; by the flat 
margins of the valves which are suddenly perpendicular to the rest of their surface ; 
by the furrowing of the ribs only on this flat marginal area; and by the elevated 
and little recurved beak. The lofty beak and the large ventral valve at once 
distinguish it from shells of the Rh. cuboides type. This beak is narrow and 
deep and has gracefully concave sides. The deltidium is long and narrow, and the 
foramen, if present, must be very small and very distant from the apex of the 
dorsal valve, but it seems to me that it was most probably closed, at least in the 
adult shell. It is, however, very difficult to obtain a specimen with the extremity 
of the beak entirely uninjured. The substance of the shell is rather thick. 
Whether Kayser is right in regarding Rh. primipilaris, von Buch, as 
differentiated from this species by its dichotomous ribs is left rather doubtful by 
Davidson. In some of our specimens the ribs occasionally dichotomise, though 
not to the extent indicated by Schnur’s figure,’ or seen in German specimens of 
Lh. primipilaris in the Woodwardian Museum, which strongly support the 
correctness of Kayser’s view. 
Should the present species ultimately prove the same as Rh. impleaa, Sowerby 
(which, however, I do not at all expect), Sowerby’s name would of course have 
priority over Bronn’s. 
8. RuYNCHONELLA IMPLEXA, Sowerby, sp. 
1840. ATRYPA IMPLEXA, Sowerby. Geol. Trans., ser. 2, vol. v, pt. 3, pl. lvii, fig. 4. 
21854. TrereBRaTULA TETRATOMA, Schnur. Palwontographica, vol. iii, p. 176, pl. 
xxii, figs. 4a—d. 
1854, — SUBCORDIFORMIS, Schnur (pars). Ibid., p. 186, pl. xxv, figs. 
6 d—gq (only). 
1 1854, Schnur, ‘ Paleontographica,’ vol. iii, p. 187, pl. xxvi, figs. 3a—e. 
¢ 
