CARBONICOLA ACUTA. 55 



A large number of genera might have been founded on the hinge-plates figured, 

 had the specimens been found in different beds ; and thus it becomes very 

 apparent that a classification of the [/mo-like forms of the Coal-measures, founded 

 on slight differences in the hinge, is not justifiable. 



Indeed, we find in this one species the edentulous forms which Prof. Amalizky 

 would designate as Naiadites, and those with cardinal teeth only, which he would 

 call Anthracosia ; and a couple of rare forms (PI. V, fig. 38, and PI. VI, fig. 2) 

 which have a trace of a posterior lateral tooth — the only specimens indicating such 

 a condition which I have seen, and which would be referred by him to Garbonicola. 



I believe that no other genus of shells, fossil or recent, exhibits such a curious 

 incoustancy in a character which is generally so regular as to be used as a mark 

 of generic import, and it is difficult to see what conditions could have induced 

 this variation. Living gregariously, with precisely the same environment, and 

 probably interbreeding very closely, it might be natural to suppose that a 

 constant type would have resulted, rather than the amount of variation which 

 really obtains. I cannot make out that any one form of hinge is peculiar to any 

 particular form of Carbonicola acuta ; nor is there much evidence to show that an 

 equal amount of variation is to be found in other districts, except that Mr. John 

 Ward, F.G.S., has a fine series of interiors of G. acuta from the ten-foot seam of 

 North Staffordshire, in which a similar amount of variation is to be seen. 



3*. Carbonicola acuta, var. rhomboidalis, nov. Plate III, figs. 13 — 21 ; Plate IV, 



figs. 1—7. 



General Characters. — As in C. acuta, but the shape is more quadrilateral and 

 less produced transversely. The dorso-ventral measurements are comparatively 

 greater, and the umbones more anterior ; while the posterior end, instead of 

 becoming compressed and narrowed, retains its tumidity till close to the posterior 

 border. The posterior slope is slight and inflated ; the posterior end very 

 bluntly truncate. The lines of growth posteriorly become bent upwards at a right 

 angle, and are crowded together. 



There is often a marked constriction of the surface of the valve, becoming very 

 wide at the inferior border ; and this may give rise to the appearance of an obsolete 

 blunted ridge posterior to it, and to the production of the posterior inferior angle 

 of the shell downwards, so that it becomes beaked. In some specimens the lines 

 of growth become oblique to the long axis of the shell (PI. Ill, figs. 13, 19). 



Dimensions : 



Dorso-ventrally . . . .26 mm. 



Antero-posteriorly . . . .41 mm. 



Laterally . . . . .18 mm. 



