The muscles are remarkably glistening and tendinous, except 

 at their expanded ends, which are soft and fleshy. They are, 

 with few exceptions, non-striated. In the posterior adductors 

 of Waldheimia transverse striations are well displayed. Their 

 impressions are often deep, and always characteristic ; but diffi- 

 cult of interpretation from their complexity, their change of 

 position, and the occasional suppression of some and combina- 

 tion of others.* There may be considerable changes in arrange- 

 ment of muscles without any important change in the internal 

 structure. Thus in Waldheimia cranium there are six muscular 

 impressions in the dorsal valve ; in W. australis there are only 

 four, the other two muscles being attached to the hinge-plate, 

 not to the valve. The valve and hinge-plate are never found 

 together, and it is, therefore, probable that in the fossil species, 

 the shells of which are found without hinge-plates, the muscles 

 may have been arranged as in W. cranium. 



On separating the valves of a recent Terehratula, the diges- 

 tive organs and muscles are seen to occupy only a very small 

 space near the beak of the shell, partitioned off from the general 

 cavity by a strong membrane, in the centre of which is j)laced 

 the animal's mouth. The large cavity is occupied by the 

 fringed arms, which have been already alluded t& (p. 5) as 

 the characteristic organs of the class. Their nature will be 

 better understood by comparing them with the lips and labial 

 tentacles of the ordinary bivalves (pp. 18, 21, and Fig. 208, p,p) ; 

 they are, in fact, lateral prolongations of the lips supported 

 on muscular stalks, and are so long as to require being folded 

 or coiled up. In Rhynclionella and Lingida the arms are spiral 

 and separate ; in Terehratula and Discina they are only spiral 

 at the tips, and are united together by a membrane, so as to 

 form a lobed disk. It has been conjectured that the living 

 animals have the power of protruding their arms in search of 

 food ; but this supposition is unlikely, since in many genera 

 they are supported by a brittle skeleton of shell, while the 



* Professor King has shown that the compound nature of a muscular impression is 

 often indicated by the mode in which the vascuUu' markings proceed from it (as in 

 Figs. 176, 181). 



