BIVALVIA. 43 
Spec. Char. P. testé equivalvi, orbiculari vel subovatd, costatd, costis 18-24 
convenis, levigatis ; sulcis equalibus, aliquando squamulosis ; auriculis inequalibus. 
Shell equivalve, orbicular, sometimes sub-ovate costated; ribs 18-24, convex and 
smooth ; interspaces about equal in breadth to the ribs, and occasionally sub-squamulose or 
imbricated ; auricles unequal. 
Diameter, 14 inch. 
Localities. Barton. 
Belgium (WVysf). 
The prevailing Pecten at Barton possesses the above characters, and it may be con- 
sidered the representative of the genus in that deposit, as the 30-radiatus is at Brackle- 
sham; and although perhaps this is not quite so variable as the Bracklesham shell, I have 
found it exceedingly difficult to fix the species. 
The generality of our specimens are nearly circular ; sometimes the diameter from the 
umbo to the ventral margin is greater than in the opposite direction, but not often so. 
Brander’s figure is a deviation in excess. I have been unable to find the type specimen 
from which his figure was taken, but, from the general accuracy of his other species, it was 
most probably a faithful representation, and in my own cabinet is a specimen somewhat 
approaching that form; so that the above name is considered to be applicable to this 
common Barton shell. 
Specimens have in general from 18 to 24 ribs, and the spaces between them equally 
divide the surface of the shell; the ribs are generally smooth, and so are sometimes the 
interspaces, but they occasionally show numerous and close imbrications, and the surface 
is covered with fine curved and diverging striz, most conspicuous between the ribs. 
The auricles are unequal in size; the one on the pedal side of the right valve is large, 
and has six or seven scaly rays, the upper, as usual, larger and more distant, and at the 
base of this is a considerable sinus for the byssus, with ridges or denticles on the body of 
the shell, varying from four to ten; the other auricle is also rayed. The muscle-marks are 
not immediately or strictly in opposition; they are situated higher up or nearer the hinge 
in the right valve, and lower down or more in the centre in the left, but it is so with 
several species in this genus.’ 
1 Some species exhibit more than ordinary variation in individuals, diverging in form far beyond what is 
shown in any proximate species ; others, living under the same (apparently) adverse circumstances, are not 
affected in a similar manner, but preserve a uniformity of character without any deviation, showing an inapti- 
tude to change under similar conditions. Where bivalve shells have displayed the greatest aberration from 
what is presumed to be the normal form or typical outline, the change seems to me rather to have been 
more towards a concentration, as it were, so that a variety of a bivalve, whose typical form would have 
proportions of length greater than height, would be most shown in a deviation from that type, by a 
reduction of length with a tendency to the orbicular. An orbicular shell in its deviations would, on the 
contrary, be rather in the direction of an enlargement or extension in its height. I might mention two 
or three Crag forms that strongly exhibit this tendency, viz., Venus casina, Cardita senilis, and Mya 
truncata; but this variation seems to be from a failure of vigour, as the abnormal form is generally 
rare. 
