358 STUDIES IN EVOLUTION 



Developmental Variations. 



General Form and Outline. — ■ Embryos of less than 3 mm. 

 in length are more nearly circular in outline than at any sub- 

 sequent period of the existence of the individual. Directly 

 thereafter the hinge-line represents the greatest diameter of 

 the shell, and the outline becomes suL-pentagonal, a feature 

 which is more apparent in young individuals having between 

 3 and 10 mm. length, as the increasing rotundity of the shell 

 with the approach of maturity has a tendency to obscure, in 

 a measure, this outline. At the earliest stage studied, the 

 dorsal valve is distinctly depressed along the median line, 

 forming a sinus containing a single plication which does not 

 reach to the beak (Plate XX, figure 14, a). This sinus grad- 

 ually becomes shallower, and the plications are increased by 

 intercalation until they are three in number (figure 14, h). In 

 the next stage all evidence of a sinus upon the anterior 

 margin disappears, leaving it even and straight as shown in 

 figure 14, c ; then the anterior edge becomes reflexed, show- 

 ing, in subsequent stages of growth, a fold where there had 

 previously been a sinus, this fold bearing at first three, then 

 five, and eventually, in the mature individual, seven plica- 

 tions (figure 14, cZ, e, /). This very remarkable reversion of 

 the fold and sinus relatively to the valves which bear them 

 is also seen in the species Rhynchotreta cuneata^ and in all 

 adult specimens may be clearly traced upon the earlier or 

 embryonal portions of the valves. 



Beak. — ■ In the first stage the ventral beak is high and 

 slightly resupinate, exposing the foramen in an inclined 

 plane. It gradually shortens and becomes erect, and when 

 the shell attains a length of 8 mm. it is bent forward, the 

 cardinal area being slightly incurved. Thereafter the inflec- 

 tion of the area increases, concealing first the deltidial plates, 

 and finally the foramen, until at maturity the beak lies 

 appressed upon the embryonal sinus of the dorsal valve. 



Foramen. — In the initial shell this is undoubtedly tri- 

 angular and free from deltidial plates. With the starting- 



