462 J. W\ Jackson— 



strikingly distinct from the massive white limestone — the Chee 

 Tor Rock — forming the summit of Topley Pike, and are probably 

 i^art of the limestones marked 8 in the Geological Survey Memoir.^ 

 A fault, with a small downthrow to the south, is present immediately 

 to the south of the section. The beds are also seen in the large 

 (upper) quarry to the west, but I have not examined these in detail. 



The presence in the Manchester Museum of Davidson's type- 

 specimens of this species has enabled me to compare critically the 

 Topley Pike examples. Davidson's types Avere presented to the 

 Museum many years ago by G. W. Ormerod, and were figured first 

 in the Quarterly Journal of the Geological Society, vol. x, 1854, and 

 later in the Monograph of British Fossil Brachiopoda (Palaeonto- 

 graphical Society), vol. ii, 1860. In the first publication three 

 speciniens [L9834:, L9836, and L9837] ^ are figured on pi. 8 (figs. 7, 8, 

 and 2-3) from, Treflach Wood, at Trefonen, south-west of Oswestry ; 

 one specimen [L9835] from the beach at Beaumaris, collected 

 by Henry Ormerod (pi. 8, fig. 6) ; and one specimen [L9838] from 

 Tidenham Chase (pi. 8, fig. 5), the figures in all cases being reversed. 

 In the second publication all the specimens, except L9834, are 

 re-figured the right way on pi. xlv, but with a somewhat different 

 legend in two cases, as follows: L9836 (= fig. 4), Llanvmynech; 

 L9837(=figs. 1-2), Trefonen; L9835 (= fig. 3), Llanymynech ; 

 L9838 (= fig. 6), Tidenham Chase. 



The Topley Pike specimens agree in every way with the above 

 specimens, both externally and internally. The fine, thread-like, 

 costation is very distinctive of the species, as well as the rows of 

 close-set pits along the furrows, when the outer layer of shell is 

 removed. But the most characteristic features are the narrow 

 area, and teeth in the ventral valve, and the remarkable convexity 

 and prominent bulging umbo. In most of the specimens the 

 ventral valve is of extraordinary thickness. When seen in situ 

 the species might be mistaken, at first glance, for a finely ribbed 

 Prodicctus giganteus, but a close examination, especially if the 

 hinge is visible, will at once reveal the difference. 



D. comoides (Sow.) differs from the present species by the 

 possession of a much deeper area, and, in consequence, a more 

 visible delthyrium. The umbonal region is also more dej^ressed.^ 



D. llangollensis is probably the species referred to in some lists 

 as Chonetes cf. comoides, at least in part. 



The occurrence of D. llangollensis in the Midland area is of 

 considerable interest on account of the presence of the species 

 in the limestones overlying the basement-conglomerate of the 

 Carboniferous Limestone in parts of the North Wales area, the 



1 The Carboniferous Limestone, etc., of North Derbyshire, 2nd ed. (1887), p. 18. 



2 Numbers in square brackets are the registered numbers in the Manchester 

 Museum. 



3 Cf. Davidson, Monog. Brit. Foss. Brach., ii, 1860, pi. 45, fig. 7, with pi. 45, 

 figs. 1-6 ; and pi. 55, figs. 6-8 ( = types of comoides), with pi. 55, fig. 9. 



