136 



ON SOME MALFORMED SPECIMENS OF 

 ANODONTA CYGNEA, L. 



By H. H. bloomer. , 



(Plate vii.) 



In the number of cases recording malformations of the shells of the 

 Unionidce, I have been unable to trace any account of the effect, if 

 any, these have had upon the soft parts of the animal. It may, 

 therefore, be of interest to give the results of the examination of three 

 specimens of Anodonta cyiinea, collected by Mr. S. P. Bolton from 

 Bracebridge Pool, Sutton Coldfield. 



For convenience sake these specimens will be referred to as A. 

 B. and C. respectively. Specimen A was collected in February last. 

 The left valve (PI. vii, fig. Ai.) of the shell shows the scars of two 

 fractures. The first commencing below the umbo, proceeds in a 

 transverse direction, but is only of a slight nature ; the second, 

 which is nearly in the centre of the valve, is situated posterior 

 to the first one, and pursues a course converging upon it. It 

 was from this second fracture, which not only destroyed a portion 

 of the shell, but also lacerated the left mantle-lobe, that the 

 malformation arose. Evidently the tendency of the mantle-lobe 

 was then to form the valve in a semicircular direction, while the 

 posterior portion followed a normal course. This apparent attempt 

 to maintain a continuous growth in two gradually diverging directions, 

 resulted in the overlapping or folding of the two adjacent parts 

 of the valve. This folding continued, and in all probability eventually 

 caused the posterior portion of the mantle-lobe to turn inwards, 

 by which means a curious in-growth of the shell was formed (PI. vii, 

 fig. A2.). The mantle-lobe when examined covered the whole of 

 the inner side of the valve and was much thicker near the line of 

 fracture. There was also a lateral process of the mantle, arising from 

 the outer side, which filled the anterior part of the cavity produced 

 by the overlapping mentioned above. 



The right valve also shows an abnormal growth, with a slight 

 inclination to follow a similar course, but does not exhibit any sign 

 of having been fractured. 



In addition to the laceration of the mantle-lobe, which the animal 

 was able to renew, the left pair of gills were permanently injured 

 (fig. i). The inhalent and exhalent canals are somewhat constricted 



