DEVELOPMENT OF SOME SILURIAN BRA CHIOPODA 353 



middle of the length, and becomes very marked in front, 

 depressing three plications, of wliich the middle one is de- 

 truded more than the others. 



Doraal valve convex, gibbous in the posterior part, with 

 the latera elevated and sub-angular; marked in front by a 

 prominent fold which begins near the beak as a depression 

 carrying four plications, of which the two central ones are 

 usually much more elevated than the other pair. 



Area high, closed l)y two triangular deltidial plates. Per- 

 foration of the ventral beak ovate, truncating the apex, and 

 limited below by the deltidial plates. 



Surface marked by from eight to ten strong, angular plica- 

 tions, which are crossed by very fine, regular, sharp, concen- 

 tric striae. iMature specimens usually measure from 10 to 

 17 mm. in length. 



Incipient Form (Plate XVIII, figures 12, 12 a). — The 

 youngest shell detected has a length of 1.5 mm., is flattened, 

 and nearly circular in outline. The dorsal valve is depressed 

 in the middle and carries four plications. The beak of the 

 ventral valve is broadly triangular, exsert, and elevated, with 

 a triangular, open area without deltidial plates. 



Developmental Changes. 



Contour. — In the earliest stages yet noticed the shell is 

 nearly circular. 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 fulness 

 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 plica- 

 tions, beginning about one-fifth the length of the shell in 

 front of the beak, widening rapidly, and becoming more 



23 



