520 MISS JOAN B. PROCTER ON THE 



nearer the median line at the median apex of the costal scutellum 

 than at the lateral apex of the vertebral scutellum, the resulting 

 plates of bone are alternately further from or nearer to the middle 

 line ; and the first, third, and fifth costal plates have advanced 

 along the proximal end of the rib so as to join the neural plates, 

 whilst the second, fourth, and sixth costal plates leave a 

 portion of the proximal end of the rib uncovered and crossing 

 the space between the incipient costal plates and the neural 

 plate 



" When the partially ossified carapace of a young tortoise is 

 dried, one cannot fail to be struck with the difference in the 

 texture and external surface of the bones which unquestionably 

 belong to the endo-skeletal vertebrae, and of those which, notwith- 

 standing their connection with neural spines and pleurapophyses, 

 are developed in the fibrous substance of the corium. These 

 nascent ' neural ' and ' costal plates ' of the carapace have a granu- 

 lar exterior and a coarse spongy texture, whilst the neural arches 

 and pleurapophyses are compact, smooth, and with a polished 

 external surface : the part of the pleurapophyses wliich passes 

 beneath and is attached to the under surface of the ' costal ' plate 

 (pis. i.-viii,), contrasts strikingly with that superimposed dermal 

 ossification. 



" The marginal plates present the same rough, coarse, 



granular character as the neural and costal plates their 



ossification has been governed by the presence of the marginal 

 epidermal scutes, and, as in the case of the costal plates, 

 by the points of jvmction of contiguous scutes ; each marginal 

 ossification is accordingly impressed by the lines indicating the 

 junction of the marginal epidei-mal scutes with each other and, in 

 the case of the middle ones, with the contiguous scutes of the 

 plastron.'" 



The whole of the foregoing evidence therefore points to the 

 fact that the costal and neural plates are of dermal origin, not 

 endo-skeletal ; and are, in fact, similar in this respect to the 

 nuchal, pygal, and marginal plates. 



Further, the corrrelation between the epidermal shield-sutures 

 and the plates beneath, observed independently in the present 

 study and previously by Owen, suggests that their growth is also 

 correlated. 



The areas of dermis beneath each shield are well marked oft' 

 from one another by a fold and an underlying groove in which 

 the shield-edges are implanted. It is probable that these 

 dermal areas grow in the same manner as their superimposed 

 shields, and that the bony plates beneath, which are of dermal 

 origin, have their growth similarly regulated. 



The following hypothesis is therefore suggested : — 



That the hony plates of the carapace develop from the dermis, 

 prvinarily in the areas of growth activity. 



