10 March, Morphogenesis of certain Pelecypoda. 



defined ridges, so that, at any rate in these species, 

 iimbonal markings are valueless for specific classification. 



Relationship amongst the British fresh-zvater Uniones. 



The study of these molluscs leads to the consideration 

 of their relationships. Evidence concerning this can be 

 obtained from two sources, viz., the early shell with its 

 umbonal markings, and the adult shell. 



I. The three types of umbonal marking in the British 

 species have already been described. The meaning of 

 these markings is shown in the development of young 

 Anodons. The ornament in the youngest Anodons con- 

 sists of two rows of " tubercles " joined into a W, later the 

 limbs widen and form a W with a short wide inner part, later 

 still, the inner limbs die away, leaving an even ridge. This 

 points to the fact that the umbonal markings of Anodon 

 pass through three phases, the first resembling U.pictoruvi, 

 the second U. tnviidus, the last being its own normal 

 form. 



A similar reading of the nepionic ornament of Anodon 

 is given in the East African forms where the markings 

 pass on to the neunic stages of the shell. Here they 

 start as " tubercles," which become prominent, forming a 

 pronounced W, which is more stronly marked where it is 

 crossed by growth lines. The inner limbs of the W 

 become less and less marked, and the outer limbs form a 

 wider and wider angle with each other, until they lie 

 practically in the same line "^ ' . 



2. In the adult shell the chief differences are : — 

 (i) The absence of teeth in Anodon. 



(2) The presence of alation in Anodon. 



(3) The absence of lunule in Anodon. 



