﻿176 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 5, 



point ; and the lines of growth are here strong, coarse ridges, decussated 

 by eighteen or nineteen radiating ribs, all of equal strength, except 

 the few posterior ones, which are a little stronger. The anterior side 

 (in the Irish specimens which I refer to this species) has a long 

 straight spine (broken off in all our specimens), and in both it is 

 divided unequally by a low ridge, which extends from the beak to 

 the margin a little below the base of the spine, and defines a prominent 

 lunette, below which the anterior slope is radiated by about seven ribs, 

 crossed by nearly straight lines of growth. Length 6£ lines. Breadth, 

 exclusive of the anterior spine, 8 lines. Depth of valves united, 5 lines. 



I believe this to be the same species with one which was obtained 

 by the Geological Survey from the Chair of Kildare, Ireland, in a 

 limestone equivalent to that of Llandeilo and Bala : we have three 

 specimens, which all vary a little from each other, one having a much 

 more decided angle on the anterior slope, the central keel flatter and 

 more elevated, and not so projecting on its forward edge, and two or 

 three of the longitudinal ribs very strong near the keel on either side, 

 the rest faint. But I do not think this could form more than a va- 

 riety, which might be named interruptus, since another specimen 

 from the same locality much more nearly resembles our shell. 



The species is easily distinguished from the P. pristis from Galway, 

 as that species has the lunette sharply depressed and regularly ribbed, 

 while the lower half of the anterior side is smooth. That shell too 

 was probably a strongly carinate one. 



Locality. Craig Head limestone. 



Maclurea , sp. Pl. VIII. fig. 7. 



M. magna, M'Coy, Rep. Brit. Assoc. 1850, 107. 



Our specimen, 3 inches broad, is very incomplete, and is only 

 figured to show the characteristic fossil of this band of limestone. 

 But there are excellent specimens, from the same locality, at Cam- 

 bridge, which agree well with ours, and show that the species had a 

 considerably smaller umbilicus than the M. magna, Hall. The shell 

 is high in proportion to its breadth, and has a character on the 

 upper surface very available for specific distinction, viz. the much 

 greater proportion of the outer whorl to the preceding one. In both 

 Hall's and Emmons' figure of M. magna, the inner whorl is more 

 than half the diameter of the outermost one — in ours it is scarcely 

 above one-third. 



In these proportions the shell before us a good deal resembles a 

 fine species common in the Trenton limestone of Canada, and which 

 I propose to name M. Logani, after the experienced geologist who 

 has brought them over. In his fine collection from the Ottawa 

 River, in company with numerous species of Murchisonia, Pleuroto- 

 maria, Turbo, Turritellal, Scalites, and other univalve shells, are 

 numerous fine Maclurece with many of the above characters ; and 

 associated with them, and in one case exactly fitting the mouth of 

 the shell itself, occurs the most singular operculum ever discovered. 



