20 G. O. Sars. 



It is evident that there mast also exist a developing stage, 

 in which there are no lines of growth at all, and that this 

 stage is followed by another with only a single line of growth, 

 the next stage having 2 such lines, and so on; but there 

 was no such young specimen in the collection. 



In order to examine the enclosed animal in its natural 

 situation within the shell, one of the valves ought to be 

 cautiously removed, which is rather easily effected by cutting 

 through the adductor muscle just at its insertion on the valve, 

 and by splitting the shell along the dorsal line. The animal is 

 thus found lying with the one side within the other valve, 

 and turning the opposite side immediately to the observer 

 (fig. 5). It may then at once be submitted to a preliminary 

 examination under the microscope with a low magnifying power, 

 and is found to be constructed much as in the genus Limna- 

 dia. As in that genus, it is fastened to the shell dorsally by 

 a sort of ligament issuing just between the umbones, and is 

 laterally connected with the valves by the strong adductor 

 muscle of the shell, occurring at some distance below the 

 umbones. Otherwise the animal is freely suspended within 

 the cavity of the shell, admitting of being moved both in its 

 anterior and posterior part, the latter being, however, the more 

 mobile. 



The part of the body extending fn front of the dorsal 

 ligament, is generally strongly deflexed, and is divided by a 

 deep dorsal depression into two well-marked segments, the 

 outer of which represents the head, the inner the so-called 

 cervical segment. The latter is evenly convex above, and 

 exhibits on each side the diverging fibres of the adductor 

 muscle of the shell, forming together a rounded area, by 

 which the animal is firmly connected with the inside of each 

 valve. 



The head (see fig. 4) is not very large, but of a somewhat 

 triangular form, being produced in front to a narroAvly rounded 



