35-± STUDIES IN EVOLUTION 



defined upon approaching the margin. The latera are nearly 

 flat. The depression, or sinus, becomes more pronounced 

 with the advance in growth, until a length of 4.5 mm. is 

 attained. After this period the four bottom plications grad- 

 ually elevate, the sinus grows shallower, and the front margin 

 of the conjoined valves becomes nearly straight. Upon reach- 

 ing a length of 9 mm., the two central plications are suffi- 

 ciently elevated to define the fold, which is hereafter the 

 principal feature of the dorsal valve. The development from 

 this point to full-grown individuals is principally directed to 

 reaching a maximum prominence in the fold, and increasing 

 the shell by increment on the lateral margins of the valves. 



The development of a sinus in the dorsal valve, its subse- 

 quent obliteration, and the final elevation of the plications 

 into a strong median fold are shown in figure 15, 1-10 of 

 Plate XVIII, in which the undulating lines represent the 

 anterior junction of the valves. 



Beak. — The apex of the dorsal valve is strong and pointed, 

 and is visible in all specimens up to a length of about 14 mm. 

 After this stage the shell becomes obese, and the consequent 

 greater inclination of the beak forces it into the foraminal 

 cavity, where it becomes hidden by the deltidial plates. 



The ventral valve is uniformly convex in all incipient 

 specimens. The sinus develops at the same period, and in 

 conformity with the fold of the opposite valve. The beak of 

 the initial shell is broadly triangular, perforate at the apex, 

 and directed outward. It gradually becomes narrower and 

 less oblique with advancing growth, and lies in the axis of 

 the shell in full-grown specimens. The initial perforation is 

 a small truncation of the beak, confluent with the open area 

 below. (See figure 16, Plate XVIII.) 



Surface Ornaments. — The prevailing number of plications 

 is eight, although it varies from seven to ten in some speci- 

 mens. The entire number appears at an early period of 

 growth, and in this respect the species offers a marked differ- 

 ence from some of the forms of Camarotoechia already con- 

 sidered, in which the plications increase by pairs. In a 



