CYRTINA. 71 



Ctrtina? carbonauius, M'Cotj, sp. PI. XV, figs. 5 — 14. 



Pentamerus carbonarius, M'Coy. Annals and Mag. of Nat. Hist., vol. x, 2(1 series, and 



British Palaeozoic Fossils, p. 442, pi. iii d, figs. 12 — 18, 

 1855. 



Spec. Char. Very variable in shape, globose, or imperfectly oval, generally longer than 

 wide, hinge line rather shorter than the greatest width of the shell. Dorsal valve semi- 

 circular, moderately convex or gibbous, with its greatest depth about the middle ; mesial 

 fold narrow, of small elevation, regularly curved or with a longitudinal depression passing 

 along its centre, and four or five smaller ribs originating near and extending to the margin. 

 The lateral portions of the valve are furrowed by numerous angular ribs, which continue 

 simple during their entire length, or become more or less subdivided from bifurcation or 

 by intercalations originating at various distances from the margin. The ventral valve is 

 moderately convex or gibbous, beak large, slightly or greatly incurved, with its extremity 

 straight, or twisted more to the one or to the other side. Area large, at times higher than 

 wide, generally concave, fissure triangular; deltidium (?). The sinus is shallow and extends 

 .from the extremity of the beak to the front, being margined by larger ribs, while smaller 

 ones occupy the intermediate space, and the lateral portions of the valve are similarly 

 ornamented to those in the dorsal one. 



In the interior of the larger valve a hinge tooth is placed on each side at the base of 

 the fissure, the dental plates converge, and after forming the fissure walls, become con- 

 joined so as to produce a single median plate or septum, which extends along the bottom 

 of the valve to within a short distance of the frontal margin. The internal details of the 

 dorsal valve remain still to be determined. 



Dimensions and proportions very variable. 



Length 15, width 16, depth 13 lines (Professor M'Coy 's type). 

 „ 17i, „ 17, „ 14 „ 

 16, „ 13, „ 10 „ 



Obs. This remarkable species has been minutely described by Professor M'Coy at p. 

 442 of his 'British Palaeozoic Fossils,' under the generic and specific denomination of 

 Pentamerus carbonarius, the author observing, moreover, that " some of the specimens so 

 nearly resemble Spirifers, that it was not until he had made sections in various directions 

 of several specimens, demonstrating the invariable presence of two narrow longitudinal 

 sub-parallel septa in the smaller valve, and the wide, extremely long, mesial septum in the 

 ventral one, with its internal divaricating portions flanking the triangular opening in the 

 cardinal area, perfectly agreeing with Pentamerus, as well as the absence of spiral 

 appendages, that he was convinced of its true genus." I must however observe that, 

 without wishing to deny the possibility of the correctness of Professor M'Coy's conclusions, 



