8 PROCEEDINGS OF THE NATIONAL MUSEUM vol.73 



in Dermochelys, and that each of these was capped by a horny scute. 

 Expansion of the large scutes was probably accomplished, not by 

 suppression of the small scutes, but by fusion with them. As the 

 small scutes were incorporated in the various dominating ones, the 

 underlying ossicles may sometimes have long persisted and have 

 produced the appearances reported in my paper of 1922. 



Attention may be called to the point of origin of the scutes of Ghelys 

 and the direction of their expansion ; also to the fact that the scutes 

 of our land and swamp tortoises develop in the same manner. The 

 vertebral scutes of Chelyd)ra and of Clemmys (pi. 2, fig. 2) have the 

 focus of their growth near their hinder border and they expand for- 

 ward and laterally. The center of growth of the costal scutes is 

 usually near the upper hinder border of the area and the expansion 

 is upward, forward, and principally downward. The focus in each 

 marginal scute is on the edge of the carapace, at the rear end of the 

 scute ; and the growth is directed forward and away from the border 

 on both the upper and the lower side of the shell. This correspond- 

 ence of the centers of origin and growth of the scutes of all the 

 scute-bearing chelonians furnishes strong evidence that these centers 

 correspond to the bony patches found on the tuberosities of Ghelys 

 and to bones in the keels of Derinochelys. 



It is interesting to observe that in the case of all the scutes the 

 growth is mostly forward, very little, or not at all, backward; and 

 it is somewhat difficult to determine the reason therefor. At pres- 

 ent it seems probable that it is connected with the growth of the 

 front part of the shell to the end of furnishing a retreat for the head 

 and forelegs. This has been accomplished principally by the for- 

 ward expansion of the nuchal, the first costal plates, and the anterior 

 2 or 3 peripherals. As the nuchal borders moved forward and 

 laterally the growth of the first vertebral scute was in the same 

 directions and little energy was left backward growth. Naturally 

 the second vertebral scute grew forward to fill the space left vacant; 

 and so for the succeeding scutes. The same explanation appears 

 to serve for the costal and the marginal scutes. 



On the lower side of the shell the anterior plastral bones expanded 

 forward and inward. The median, or interplastral, row of epithe- 

 cal bones, with their scutes, were early suppressed, so tliat the 

 definitive scutes were supplied from the bones of the lateral plastral 

 keels. As a result we find that the horny scutes have their centers 

 of growth on the outer and rear borders. 



In most Thecophora there are left few or no indications of the 

 inframarginal keels of DevTiwchelys except perhaps the scutes at the 

 ends of the bridges. In species of Baena there is on each bridge a 

 row of large inframarginal scutes. Where such scutes are missing 



