140 CARBONIFEROUS LAMELLIBRANCHIATA. 



Above the hinge is a very narrow elongate groove, posterior to the umbo, for the 

 insertion of the external ligament. 



The valve is irregularly convex from above downwards and before backwards, 

 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 mnscle-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 backward. 

 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 accentuated. Shell of moderate thickness. 



Dimensions. — PI. XXII, fig. -1, measures — 



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. Isevirostris (Port- 

 lock, sp.), and the two species are not likely to be mistaken one for the other. The 

 figure of Ct. Haiti* has a much greater resemblance, and, indeed, it is possible that 

 the species may be identical ; but in the absence of any specimens of the Spanish 

 shell I have hesitated to refer the Penton shells to that species. They seem to 

 me to be more transverse, more pointed posteriorly, narrower from above down- 

 ward, and neither in the figure nor in the description of Barrois's shell is any 

 mention made of the angular ridge parallel to the hinge-line which is present in 

 Gt. pentonensis. 



The dimensions of Gt. Halli, Barrois, are here given for comparison — 

 Antero-posteriorly . . . .32 mm. 



Dorso-ventrally . . . .19 mm. 



Genus PaLjEONEilo, Hall, 1870. 



Nuctjlites, Conrad, 1842. Jouru. Acad. Nat. Sci. Philad., vol. viii, p. 249. 

 Leda, Stevens, 1858. Amer. Jouru. Sci., ser. 2, vol. xxv, p. 262. 



1 Barrois, " "Recherches sur les Terrains anciens des Asturies & de la Galice," ' Mc'ui. Soc. Gcol. 

 Nord.,' vol. li (1882), No. 1, p. 339, and pi. xvii, fig. 3. 



