350 STUDIES IN EVOLUTION 



maturity, all have the same unspecialized starting-point. 

 Hence the fact that some of these variations have not shown 

 a complete series of immature stages must be due to the 

 insufficiency of the material, rich as it has been. 



Limiting these considerations now to the normal form with 

 two plications in the ventral sinus, it may be seen that the 

 initial shell is smooth, and obcordate in outline, with beak erect, 

 while the mature shell is strongly plicate, strongly ovate, and 

 with the beak sharply incui-ved. The transition from one 

 extreme to the other is through stages of growth between the 

 limits .65 X.54 mm. (initial) and 12 X 12 mm. (average adult) 

 In growth-stages below 7x5 mm. dimensions, the shell is very 

 depressed-convex, the dorsal valve up to about this point 

 retaining a low, broad, median depression, accompanied by a 

 similarly low and broad median elevation on the opposite 

 valve. It is not always possible to determine with accuracy 

 how many plications are carried by this embryonic fold and 

 sinus, on account of not being well limited; but their 

 eventual revei'sion, in the adult shell, into sinus and fold 

 respectively, marks the feature as an interesting one, to 

 which attention is called more at length in the description 

 of the species Rhynchotreta cuneata and Atrypa reticularis. 

 Rare instances occur of individuals assuming all the characters 

 of maturity before attaining a length of 6 mm., and from 

 this point up to the normal size for adult growth, mature 

 dwarfs are frequently found. 



Beak. — In the initial shell the beak of the dorsal valve is 

 rounded and inconspicuous, and so remains in all stages of 

 growth. In the opposite valve the beak is at first high, 

 erect but not acute, the cardinal margins sloping abruptly ; 

 and with increasing age the beak becomes fuller, more and 

 more incurved at the apex, but is never closely procumbent 

 upon the dorsal umbo, as is the case at maturity with most of 

 the plicate species here described. 



Foramen. — At the outset the pedicle-aperture is narrowly 

 sub-triangular, reaching to and encroaching upon the apex, 

 free of deltidial plates and with the lateral margins unthick- 



