8 March, Morphogenesis of certain Pelecypoda. 



The disappearance of this part would be accelerated 

 by two facts. 



{a) Its position would be a hindrance to an animal 

 creeping through mud or water, and therefore 

 any tendency to lose it would be advantageous 

 to the individual. 



{b) It is formed during early growth of the shell, and 

 is therefore thin, and not likely to be especially 

 strengthened as it contains none of the body. 



The extreme sensitiveness of the Unionid shell, as 

 shown by the study of the British fresh water species, 



makes it quite impossible to classify these Lamellibranchs 

 on form alone. If nothing but the shell were known, 

 distinct species would be made out of the typical Ossing- 

 ton forms {Fig. F of Plate), the large, tumid, Birmingham 

 types {Fig. A of Plate), the elongated Marple specimens 

 with the forward throw of the umbo {Fig. B o{ Plate), the 

 long Burnley shells with the normal umbo {Fig. C of 

 Plate), and the platyrhyncoid Foss and Lake Rudyard 

 forms {Figs. E & D of Plate), — leaving out of account the 

 distinctions that could be drawn in these types, based on 

 a study of umbonal markings. 



Ornament in British Uniones. 

 The ornament in the British Uniones is confined 

 entirely to the nepionic stage, and is therefore probably 

 of phylogenetic importance. 



The markings are of three types : — 



(i) U. pictoriim type, which consists of two divergent 



rows of V-shaped " tubercles," which may be 



joined. 

 (2) U. tumidus type, where the inner limbs of the 



" V's " are joined and shorter than the outer ones, 



