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 strise 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 slightly arched. Hinge-plate triangular as in G. 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 (PI. 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, PI. 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, PI. VII, from the matrix, 

 and cleaned out the hinge-plates, finding them to be exactly on the plan of that 

 shown in PI. V, fig. 1. I have not been able to get at more than two interiors, 

 but these do not vary at all. On PI. 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. 



