DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 371 



single plication on one side of the median plication of the 

 dorsal valve. 



A tendency to obesity is often manifested by the shell, at 

 or before reaching the average dimensions of maturity, when 

 it may be supposed that the full growth of the individual has 

 been attained. This obesity is produced by a rapid thicken- 

 ing of the shell at the margins, making the anterior face 

 truncate and forcing the ventral beak over the dorsal in the 

 same manner as if the valves were forced to open along 

 the hinge. It is, therefore, only in individuals which have 

 reached this obese condition that the ventral beak is incurved. 



Developmental Changes. 



The character of the primal or elemental shell may be seen 

 from a single example (Plate XIX, figure 18), in which the 

 plications are abruptly developed at a distance of 1.5 mm. 

 from the apex, and, presumably, that portion of the shell 

 within this limit represents approximately the size of the 

 original embryonic shell. This 1 portion of the individual is 

 quite smooth, and shows but a trace of the median fold and 

 sinus. As already observed, there is a marked permanency 

 in the surface features of the species from early youth to 

 maturity. The smallest individual obtainable bears, as in 

 the mature condition, six plications on the ventral and five 

 on the dorsal valve, though those near the hinge-line are quite 

 faint. 



The beak is prominent and exsert, except in obese shells, 

 where it is incurved. In the earliest stage where the char- 

 acter of the foramen is well preserved the individual has a 

 length of 4 mm. and a width of 3.5 mm. Here it is seen to 

 be elongate-oval, the deltidial plates having formed to such 

 a degree as to be in contact with each other and to have 

 anchylosed, so that the median suture is detected with diffi- 

 culty. The lateral sutures always remain distinct even to 

 maturity, and it is evident that the union of the plates with 

 the shell along these joints has not been as firm as in many 

 species, as it is not infrequently found that the plates have 



