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 Unio-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 Pl. 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 Carbonicola. 
I believe that no other genus of shells, fossil or recent, exhibits such a curious 
inconstancy 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 C. 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. ITI, figs. 13, 19). 
Dimensions : 
Dorso-ventrally : u : . 26 mm. 
Antero-posteriorly : : ‘ . 41 mm. 
Laterally : : : . 18 mm. 
