358 STUDIES IN EVOLUTION 



Developmental Variations. 



General Form and Outline. — Eillbiyos 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 sub-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, 5). 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, (?, 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 tlie 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- 



