14 March, Morphogenesis of cert am Pelecypoda. 



{b) The most definite phyloephebic ornament is seen 

 in the forms from the rift lakes of E. Africa. 

 U. nyassensis ornament begins as two diagonal 

 lines joined by a concentric bar, but later alters 

 into a V. Specimens from lake Miverw, are purely 

 W marked throughout. 



In Qicadrida lachrymosa. {=U. lacJirymosus 

 [Lea]) the ornament is carried on from nepionic 

 stages, dying away in old age, but is more or less 

 distinct throughout. 



In U. ligainentimis the ornament consists merely 

 of strongly marked growth lines, and is clearly 

 phylogerontic. 



(c) Q. PusHdosa. (t/./;/j'//^^j'/i;i" [Lea]) starts life as a 

 smooth shell, in extreme old age it developes 

 irregular pustules, the ornament here is purely 

 phylo-hyostrophic. 



I hope to obtain further evidence on the nature of 

 Unlonid ornament. 



The PJiylogeny of the Unionidce. 

 It is impossible to study any branch of the UnionidjE 

 without considering their phylogeny. Two possible lines 

 of descent are admitted, — the one through the Cardiniidae, 

 and so in relation to the Carbonicolas. This view, accord- 

 ing to Zittel's (Zittel, 1900) summing up, is the general 

 one. The other, and older view, gives them a Trigonid 

 ancestry. Several points seem in favour of this connection 

 with the Trigonias. 



(i) Dentition. 



(2) Form of the shell. 

 The typical Trigonid dentition consists of short 

 lateral teeth just in front of and behind the umbo, with 

 the antero-laterals (pseudo-cardinals) supported by a 

 strong buttress. 



