SILURIAN BRACHIOPODA 49 



DEVELOPMENTAL CHANGES 



Contour. In the earliest stages yet noticed, the shell is nearly cir- 

 cular. At a length of 2 mm., it is broadly oval, and at 2.5 mm., it is ovate. 

 The beak in the next advanced stage is more elongate, and when the length 

 of 4 mm. is reached, the shell has a decidedly triangular or cuneate form, 

 which becomes more pronounced up to maturity. All the young and 

 adolescent shells are depressed, the characteristic fullness of the valves not 

 being developed until after the assumption of the features of maturity, 

 and when the shell approaches its normal size. 



Fold and Sinus. The smallest individual shows a slight depression 

 in the dorsal valve, co-existing with the plications, beginning about one-fifth 

 the length of the shell in front of the beak, widening rapidly, and becom- 

 ing more 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 gradually elevate, the sinus grows shallower, and the front 

 margin of the conjoined valves becomes nearly straight. Upon reaching 

 a length of 9 mm., the two central plications are sufficiently 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 prin- 

 cipally directed to reaching a maximum prominence in the fold, and increas- 

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



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

 eration, and the final elevation of the plications into a strong median fold, 

 are shown in figures 15, i-io of plate 4, 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. 



7 



