BIYALYIA. 43 



Spec. Char. 1\ testa cequivalvi, orbicular i vel subovatd, costatd, costis 18-24 

 convexis, lavigatis ; sulcis aqitalibus, aliquando sqiiamulosis ; atiriculis inaqtiatibus. 



Shell equivalvc, 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, \\ inch. 



Localities. Barton. 



Belgium (Ngsf). 



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-radiat/(S 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 striae, 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 iu the centre in the left, but it is so with 

 several species in this genus. 1 



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, aud Mya 

 truncata ; but this variation seems to be from a failure of vigour, as the abnormal form is generally 

 rare. 



