8 CARBONIFEROUS LAMELLIBRANCHIATA. 



that some better example must have been used. The example in the Griffith Collec- 

 tion is a left valve, and M'Coy's figure appears as a right valve, but it must be 

 remembered that M'Coy never reversed his figures on the stone, so that the opposite 

 valve is always represented. 



The posterior edge of this shell was remarkably thin, and often only the strong 

 rolled hinge remains, so that the shell appears to have a long linear process 

 posteriorly, which is not the case in perfect specimens. 



P. angustatas is much smaller, more compressed, and less triangular than 

 F. latus. It is not a common shell, and is always found in shaly beds, pointing to 

 its mud-haunting habits. This species is found at a much lower horizon in Scotland 

 than in England, but it is too rare a shell to construct a curve showing the 

 successive periods at which it reached definite localities on its way south, as can be 

 done in the case of various other Lamellibranchs (vide ' Quart. Journ. Geol. Soc.,' 

 vol. Ivii, p. 380). I have little hesitation in referring Pteronites naviformis and P. 

 sitbventricosns, de Koninck, to this species. 



Ptebonites latus, M'Coij, 1844. Plate V, figs. G and 7. 



Pteronites latus, M'Coy, 1844. Synops. Garb. Foss. Ireland, p. 81, pi. xiii, 

 fig. 7. 



— — Morris, 1854. Cat. Brit. Foss., 2nd edit., p. 181. 



— — Bigshy, 1878. Thesaur. Devonico-Carboniferus, p. 295. 



— — Etheridge, 1888. Brit. Foss., pt. 1, Palaeozoic, p. 276. 



Specific Chdvactrrs. — Shell below medium size, compressed, transversely tri- 

 angular. The anterior end is obsolete, and the front part of the valve is somewhat 

 turned, the lower margin adpressed to form the cavity of the valve. The inferior 

 Ijorder is slightly concave at first, then descends downAvards and back^vards, where 

 it becomes convex and broadly curved, passing round into the posterior margin 

 with a single sweep. The posterior margin is truncate, very slightly sinuous above 

 the postero- superior angle, a right angle. The hinge-line is long and straight ; the 

 umbones are small, pointed, and terminal (subterminal in cast of the interior). 

 The anterior part of the valve is narrow and convex, the posterior flattened and 

 expanded. The posterior wing is relatively large, triangular, compressed, marked 

 off from the rest of the valve by a straight shallow sulcus, which passes obliquely 

 downwards and backwards from the umbo to the posterior border. 



The hinge is edentulous, with two longitudinal grooves posteriorly. 



Exterior. — The surface is almost smooth, ornamented with obscure flattened 

 ribs and very shallow sulci, which are parallel to the margins. 



