Vol. 56.] PALCEONEILO IN BRITISH CARBONIFEROUS ROCKS. 47 



interrupted below the umbo, and by having no shelly process (the 

 clavicle) separating the anterior adductor muscle-scar from the rest 

 of the valve. Ctenodonta, however, is much more nearly related to 

 Palceoneilo, but the former is nearly equilateral, and has no vertical 

 comb-like hinge-teeth just below the hinge, neither does it possess 

 the characteristic radiating sulcus, on the dorsal slope, or the well- 

 marked escutcheon. 



Beushausen has, however {op. supra cit.), referred all the Devonian 

 shells of Ehenish Prussia to Otmodonta, considering that Palceoneilo 

 is nothing more than a subgroup of that genus. (Ehlert (op. supra 

 cit.), on the other hand, considers Palceoneilo to be generically distinct 

 from Ctenodonta. Whidborne l refers certain shells to Otenodonta 

 (Palceoneilo). 



The following is a formal description of the species to which I 

 give the name Palceoneilo carbonifera. 



Paljsoneilo carbonifera, sp. nov. (Figs. 1-3, p. 48.) 



Specific characters. — Shell of more than medium size, trans- 

 versely ovate-rhomboidal, oblique, very inequilateral, gibbose. The 

 anterior end is very small, gibbose, narrowed from above downward, 

 with its margin rounded. The inferior border is rounded in front, 

 almost straight posteriorly, forming a well-marked, slightly obtuse, 

 but gently rounded angle with the posterior border. The latter 

 margin is sinuous, convex above and concave below, the upper 

 portion being the larger. The postero-superior angle is very wide. 

 The hinge-line is arched, though the upper margin of the valve, 

 posterior to the umbo, appears straight, and is somewhat depressed 

 as it passes backward. 



The. umbones are large, tumid, incurved, and markedly twisted 

 forward, contiguous and elevated, placed very far backward, and 

 much excavated in front, but there is no true lunule. Passing 

 downward and backward from the umbo obliquely to the postero- 

 inferior angle is a blunt ridge which separates the dorsal slope from 

 the rest of the valve. In front of the oblique ridge the valve 

 is convex from above downward, and below backward, the dorso- 

 ventral curvature being much greater than the transverse. There 

 is a marked flattening, or broad shallow sinus, in front of the ridge. 

 Immediately above and posterior to the ridge is a well-marked sulcus, 

 commencing as a narrow groove just behind the umbo, but becoming 

 deeper and broader as it approaches the posterior margin, to the 

 concavity in the border of which it corresponds. Above this radiat- 

 ing sulcus the dorsal slope swells, so as to become markedly convex, 

 but this convexity is separated from the upper margin of the valve 

 by a shallow groove forming the outer limit of the escutcheon. 

 The escutcheon is large and well marked ; it is bounded internally 

 by the narrow elongate groove for the external ligament, and 

 externally by a slight curved ridge which starts from the umbo, 

 and, curving outward, at first gradually approaches the margin 

 coalescing with it near the postero-superior angle. 



1 Monogr. Palseont. Soc. ' Devonian Fauna,' vol. iii, pt. i (1896) p. 98. 



