62 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
The umbones appear to rise gradually from the whole surface of the shell, and 
are blunt, raised above the hinge-line ; excavated anteriorly, and thinned internally 
by the encroachment of the lunule, which is moderately deep, widens anteriorly, 
and is roughly marked by striz continuous with the lines of growth which pass 
into it from the surface of the shell. The external ligament is prominent and 
erect, extending half the way from the umbones to the posterior end. The 
hinge-line is shghtly arched. Hinge-plate triangular as in C. acuta, the normal 
form ; but, owing to the more quadrate and stumpy form of the shell, this part of 
the plate, posterior to the cardinal tooth, is placed at an obtuse angle to the 
anterior portion. 
Valves compressed with an almost obsolete, broad, oblique sinus, most 
apparent at the lower border. The posterior slope is dilated and obtuse. Shell 
thick. Muscle-scars normal. 
The surface is rough with coarse lines of growth. Periostracum thick and 
much wrinkled. 
Dimensions (Pl. VII, fig. 18) : 
Antero-posteriorly : : : . 35 mm. 
Dorso-ventrally : ‘ : . 25 mm. 
Laterally . . 16 mm. 
Localities.—The roof of the Ten-foot Coal, and the horizon of the Cockshead 
Rock, Hulme Colliery. Ivy House Colliery, Bucknall. Whitfield Colliery. All 
in the Lower Coal-measures of North Staffordshire; Middle Coal-measures of 
Oldham. 
Observations.—The form of this species appears to be less variable than most ; 
and although fig. 22, Pl. VII, appears to differ in form from the others, it will be 
noted that the shell is imperfect at the posterior-inferior angle. 
Since having the plates printed I have lifted fig. 16, Pl. VII, from the matrix, 
and cleaned out the hinge-plates, finding them to be exactly on the plan of that 
shown in Pl. V, fig. 1. I have not been able to get at more than two interiors, 
but these do not vary at all. On Pl. XI, fig. 1, is the figure of the hinge-plate 
of fig. 16, which will be of interest, as it is very rarely that the hinge-plates of 
two opposing valves can be studied. 
The lines of growth are more rough, irregular, and deeper cut than in other 
forms; and this point, taken in connection with the shape, will serve as diagnostic 
in determining the species. 
This species occurs at only one horizon in the North-Staffordshire Coal-field, 
but in great abundance, whole beds being made up of it. It is rare in certain 
places to find the valves in contact, showing that the shells were probably washed 
into little bays after death. In these places are to be found occasionally a pair of 
valves opened out as in Pl. VII, fig. 16. 
