142 CARBONIFEROUS LAMELLIBRANCHIATA. 



Hammerton Hall, Slaidburn, Yorkshire," which I think means that the shells 

 occnr above the massif of Mountain Limestone, and at the base of the Pendleside 

 Series, at which horizon the allied genus Gtenoclonta also occurs. 



It is therefore curious that a genus so well developed in Devonian times should 

 appear at the top of the Carboniferous Limestone Series, there being no trace of 

 its existence in intermediate beds. It is also noteworthy that the species attains 

 considerable size, and is remarkably well developed, the shell possessing all the 

 distinctive characters of the genus. 



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

 Palmoneilo carbonifera. 



Pal^oneilo CARBONIFERA, Hind, 1900. Plate XXII, figs. 8, 8a, 8b. 



Pal.eoneilo caebonifera, Hind, 1900. Quart. Journ. Geol. Soc, vol. lvi, p. 47, 



figs. 1, 2, 3. 



Specific Characters. — Shell of more than medium size, transversely 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 radiating 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 



