512 MISS JOAN B. PROCTER ON TIIK 



fenestra, represented in the adult by a small fontanelle, and from 

 the sixth by a narrow one, absent in the adult ; the latter fenestra, 

 however, is divided into two by the contact of the posterior corner 

 of the end of the dilated sixth with the seventh costal, about 3 mm. 

 from the marginal plates. This is caused by the relative positions 

 of the implicated ribs, which become closer together posteriorly 

 as the vertebrpe become progressively shorter. Foi' this reason, the 

 eighth costal plate in this specimen completely roofs over the ninth 

 and tenth (abnormal) ribs, the intervening space between them 

 being no broader than the entire second costal plate (compare 

 text-figs. 18 and 15). 



Therefore, in the first instance (42 mm.) it is the central portion 

 of the carapace which is the most developed ; later (60 mm.) the 

 plates are more or less equally developed all over, with the exception 

 of the neurals, which are progressively enlarged posterioidy. Con- 

 tinuing up the series (71, 81, and 89 mm.), the first, second, fourth, 

 sixth, and eighth costal plates expand in length until they form 

 sutures with the marginals ; the third and fifth become the slender 

 rib-like plates of the adult form ; and the posterior portion of the 

 carapace from the seventh costal to the pygal plates becomes 

 roofed in with bone by the expansion of the seventh and eighth 

 costal plates. 



The anterior part of the carapace in the adult, therefore, is 

 arrested at an earlier stage in its development than the posterior. 



The plastron. 



At 42 mm. the plastron is in the initial stage of development 

 (vide text-fig. 19) ; all nine bones are present, but very small, 

 thin, and for the most part widely separated from each other. 

 The epiplastrons are merely small strips of bony deposit beneath 



Text-figure 19. 



Plastron of newlj^-liatcbed j'oung, from without (nat. size). 



the anterior portion of the pectorals. The entoplastron is better 

 developed, and subtriangular in shape. The hyoplastrons are 

 ragged in outline, and not in contact with epiplastrons or 



