12 March, Morphogenesis of certain Pelecypoda. 



which cannot have occurred amongst the British Uniones 

 and which must have been of a secondary character. 



3. The lunule, which, absent in Ancdon, and but 

 feebly represented in some Uniones, is well developed in 

 some American forms and even in early, probably Jurassic, 

 Margaritanas. 



In the study of the Unionidae, Mr. J. W. Jackson has 

 noticed that it disappears with the loss of the pseudo- 

 cardinals. This absence, then, in A. cygnca, is due to the 

 loss of these teeth, and with it the necessity for great 

 width in the anterior part of the shell. This last difference, 

 then, is of a secondary character. 



Amongst the British Unionida^, therefore, U. iuviidus 

 appears to retain the most primitive features both in 

 nepionic and ephebic stages, and so to resemble the parent 

 stock most closely. Anodoji is the most highly specialized. 

 U. pictornin, though occasionally showing a more strongly 

 marked W than either of the other two, yet usually has 

 the limbs of the W undeveloped, thus leaving unconnected 

 " tubercles." 



Pictoruvi and Anodon then, represent divergent cases 

 of degeneration from iuviidus. In the former case the 

 limbs of the W have died away, in the latter the inner 

 limbs have become so stretched out as to form a more or 

 less concentric line. 



The more prim ive ephebic characters of tumidus are 

 seen in its better developed teeth and marked anterior 

 buttress. 



The gaps between Anodo?> and U. f?n//idus can be filled 

 in from the American fauna. 



Ornament in Foreign Unionidce. 



The history of ornament in the Unionida^ since Triassic 

 times has been one of degeneration from fully ornamented 



