Vol. 55.] CAEBONIFEROTJS LAMELLIBRANCHIATA. 369 



1 mile north-west of this locality, and at Peter's Crook Quarry, 

 about I mile to the east on the English side, in both of which 

 are seen two thick limestones separated by a richly fossiliferous 

 marine shale which has a fauna largely identical with that con- 

 tained in the shale in the river-bank. This bed is rich in gaste- 

 ropoda, the genera Murchisonia, Macrocheilus, Pleurotomaria, Looco^ 

 nema, and Euomphalus being represented in it. Belleroplion Urei 

 and B. decussata also occur, together with Orthoceras, Fenestella^ 

 brachiopods, crinoids, and Archceocidaris. Productus giganteus 

 occurs in the limestones, and therefore affords strong evidence 

 that the identification of the horizon with that of the Hurlet 

 Limestone is correct. 



Ctenodonta penton-ensis, sp. nov. (PL XXY, figs. 1-3, 3a & 4.) 



Specific Characters. — Shell transversely and triangularly 

 ovate-acute, moderately gibbose, very inequilateral. 



The anterior portion of the shell is about one-third of the valve, 

 and is moderately swollen, its border being elliptically curved. The 

 inferior border is extended, and is very convex. The posterior border 

 is exceedingly small and bluntly pointed, much narrowed by the 

 approximation of the upper and lower borders. The hinge-line is 

 arched, especially in front, but becomes straight, extended, and 

 depressed posteriorly. 



The umbones are moderately swollen, incurved, contiguous, 

 elevated, forming the highest point of the shell, and excavated in 

 front, but there is no real lunule. Above the hinge is a very 

 narrow elongate groove, posterior to the umbo, for the insertion of 

 the external ligament. 



The valve is regularly convex from above downward and before 

 backward, and most specimens show that an angular ridge parallel 

 to, but at a higher level than, the hinge-line passes from the 

 umbones backward to the posterior end, indicating a bending of 

 the valve on itself. 



Interior. — The muscle-scars are not exposed in any of the 

 specimens yet obtained. The hinge-plate consists of two rows of 

 small triangular teeth which meet at an obtuse angle beneath the 

 umbo. The anterior set are fewer and larger than the posterior, 

 about six in number, the teeth becoming smaller from before back- 

 ward. The posterior row contains about eighteen to twenty teeth, 

 which increase in size from before backward. 



Exterior. — The surface is covered with well-marked concentric 

 lines of growth, one of which occasionally becomes much accentu- 

 ated. Shell of moderate thickness. 



Dimensions. — Antero-posteriorly, 30 mm. ; dorso-ventrally, 

 17 mm. ; elevation of valve, 7 mm. 



Locality. — A bed of marine shale below the highest limestone, 

 Penton Linns (Dumfriesshire). 



Observations. — Ctenodonta pentonensis differs widely from Ct. 

 sinuosa (de Ryckholt), and the two species are not likely to be 



Q.J.G.S. No. 219. 2 b 



