5l6 



MISS JOAN B. PROCTER ON TS^ 



dermal bones of the carapace is well advanced, showing most 

 clearly how the carapace of the adult is formed. The anterior 

 and median portion is developed almost as much as in the adult, 

 but posteriorly large fenestrae occur beneath the fourth costal 

 and fifth vertebral shields, which (fenestrse) are almost entirely 

 roofed in with bone in the adults. 



The nuchal and marginal plates are fully formed. The neurals 

 are of the adult pattern, but they are narrow, thin, and feebly 

 developed anteriorly, becoming progressively broader posteriorly, 

 the seventh and eighth are the broadest, followed by a much 

 narrower first pygal. The first pygal is not well developed, and 

 is separated from the last costal plate on each side by a very wide, 



Text-figure 18. 



Carapace of young specimen 60 mm. in length, from witiiin (^ nat. size). 



subtriangular fenestra, which is represented in the adult by a 

 small one of ragged outline in the last costal piate itself. 



From the study of this one stage it is easy to reconstruct the 

 development of the bony carapace of the adult, and, by logical 

 conclusion, to suggest a manner in which a solid carapace can be 

 formed. 



Costal plates one to six are similar in form and arrangement 

 to those of the adult, excepting that they do not reach the mar- 

 ginal rib by about 3 mm. In the case of the first, the shortness 

 of the plate causes a small fenestra to be discovered between 

 the underlying lib and the anterior marginal plates. 



At this stage one sees the relative positions of true ribs and 



