﻿STRKPTORIIYNCIIUS. 



81 



in different examples of Sir. umbracidum, as well as of the Carboniferous Sir. crenistria. 

 In some specimens the intermediate space between each of the two larger ribs is partly 

 occupied by a smaller rib, which, commencing to appear at variable distances from the 

 beaks, extend to the front ; while in other examples the intervening ribs are irregular in 

 their respective widths, and can hardly be distinguished from the principal ones. 



It appears to be very difficult to obtain from our British Devonian rocks specimens of 

 Str. umbracidum with the shell-surface perfectly preserved; but when so, and as is often 

 visible on those from the Eifel, the ribs are ornamented, as we have already described, while 

 the intervening spaces are more or less finely transversely striated ; but when the outer 

 shell-surface has not been preserved, the ribs and the interspaces appear smooth. 



In our Upper Devonian rocks of North Devon we unfortunately meet with but im- 

 pressions and internal casts, and these certainly appear undistinguishable from similar 

 Carboniferous casts and impressions of Strept. crenistria and its variety arachnoideus ; and 

 it is only in the Middle Devonian limestones of Lummaton and Hope's Nose, near Tor- 

 quay ; East Ogwell, and Woolborough quarry, near Newton Abbot ; or in the neighbour- 

 hood, that we occasionally meet with specimens sufficiently perfect to be identified with 

 Strept. umbraculum of the Eifel. 



The so-termed Orthis tenuistriata, Sowerby, pi. XIX, fig. 3, was established on a 

 single specimen from Moreback, now in the collection of the Geological Society, and which, 

 to my eye, has all the appearance of a small individual of Str. umbraculum. In shape it is 

 semicircular, with a straight hinge-line equal to the width of the shell. The dorsal valve 

 is convex, while the ventral one is slightly so only at the beak, becoming afterwards flat- 

 tened and slightly concave at the margins. The surface of the valves is striated in the 

 manner we have described for Str. umbraculum. The area in the specimen is obscured by 

 matrix. 



Streptorhynchtjs crenistria, et var. arachnoideus, Phillips. PI. XVIII, figs. 4 and 7. 



Sfirifera crenistria, Phillips. Geology of Yorkshire, vol. ii, p. 21 C, pi. 9, fig. 6, 



1836. 



Streptorhynchus — Dav. Mon. Brit. Carb. Brach., p. 125, pi. xxvi, figs. 1 — 6, 

 pi. xxvii, figs. 1 — 5, and pi. xxx, figs. 14 — 16, 1861. 



Spirifera arachnoidea, Phillips. . Geology of Yorkshire, vol. ii, p. 220, pi. 11, fig. 4, 



1836. 



As already observed, casts and impressions agreeing with the Carboniferous form occur 

 plentifully in the Upper Devonian grits of many localities in North Devon, as well as of 

 Cornwall, such as at Braunton ; Upscott, parish of Pilton ; the neighbourhood of Barn- 

 staple, and in' Middle Devonian at Ilfracombe, &c. 



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