1 6 March, Morphogenesis of certain Pelecypoda. 



(i) The entire absence of a lunule in Trigonias seems 

 to be associated with the position of the teeth. 



As aheady shown the disappearance of the lunule is 

 associated with the loss of anterior lateral teeth. Its 

 appearance may be connected with the forward shifting of 

 these teeth. In Trigonia they lie close under the umbo, 

 where the shell is naturally wide, but after they have 

 moved forward, they get into a region where the sides of 

 the shell lie more closely together, especially if the animal 

 becomes elongated, and where some modification is needed 

 to give room for the teeth. This is done by the lunule. 

 Also the lunule may or may not be present in Uniones. 



(2) The link forms between non-buttressed and 



Unionidae and strongly buttressed Trigonias 

 have already been given. 



(3) The absence of an accessory pedal scar in 



Trigonias is paralleled by its absence in Quad- 

 rula. Mr. J. W. Jackson has suggested that its 

 absence is due to the shifting of the muscle from 

 the buttress, where it was originally attached, on 

 to the shell. 

 The similarity in the essential shell characters, and 

 possibly in the ornament, together seem to point con- 

 clusively to close relation between the Unionidae and the 

 Trigonidae. This relationship can only be that of a 

 common ancestry from a pre-Triassic, pre-Trigonid stock, 

 as the Unionida: are known from Triassic times as well as 

 the Trigonidas. The very distinct ornamentation amongst 

 the Liassic Trigonida^, paralleled in the Triassic 

 Myophorias and Trigonias, puts their convergence back 

 into Permian times at least. I think it probable that the 

 study of ornament in the Unionida; will show the presence 

 of as distinct, and possibly as ancient, lines as in their 

 relatives. 



