AXOMIA. 43 



are never exactly of the same size, nor do they preserve their 

 relative positions in any of the varieties, in consequence of the 

 animal dividing the muscular mass into three fasciculi of fibres, 

 varying in quantity, thus altering the shape and relative distance 

 of one portion from the other ; therefore these impressions are 

 of little distinctive value. 



Finally, as regards the animal, it may be observed, that in 

 this singular unsymmetrical genus, even its organs display, 

 like the shell, varieties of form ; this arises from the entire 

 animal being deposited in the convex valve ; it only rests on 

 the flat one, and the organs, in consequence, vary with the 

 ever-varying figure of the upper valve. 



I will now make some remarks on the various markings 

 and aspects of the shells of the so-called British species, of 

 which I have examined above a thousand. And as regards 

 their general shape, they vary from all grades of subcircularity 

 to every subtriangular form. I have seen on the same Pecten, 

 on the shells of which genus the Anomia are oftener fixed 

 than on any other, two individuals of the typical dirty- white 

 A. ephippium, the one displaying its strictly squamous cha- 

 racter, and marked not only with ribs, but the vaulted and 

 arched spines of the Pecten ; the other, in contact, without a 

 rib or spine, and only showing the regular squamae of increase. 

 I have also seen the A. ephippium with half the transverse 

 portion of the shell of a perfectly squamous character, and the 

 basal half ribbed and spiny, and vice versa. The same inci- 

 dents are seen in the rosy purplish A. cepa and bright yellow 

 A. electrica of British authors, which are mere deviations of 

 colour from the type. 



As for A. striata, otherwise the A. undulata of some authors, 

 which is certainly the most aberrant form of the A. ephippium, 

 the distinctness of which has been insisted on from the iri- 

 descent green colour of the inside of its valves, the radiating 

 muscular impressions and the intense vermilion of the ani- 

 mal, I have to observe that in young specimens, of one and a 

 half inch diameter, having the anastomosing radiating striae 

 found on the smooth insides of the Pecten max'mius, I have 

 noticed these characters to be by no means constant, having 



