CARAPACE AND PLASTRON OF THE CHELONIAN REPTILES. 
167 
common centre, yet their essential homologies are neither thereby destroyed nor 
much masked. 
The unimportant, one might almost say accidental character of connation, in re- 
gard to the neural plates of the carapace, is shown by its absence in at least one- 
fourth of the series of those plates. Rathke admits that the first or nuchal plate, 
the ninth, tenth, eleventh and twelfth of the median series of plates of the carapace 
are developed from independent centres in the substance of the corium, and are there- 
fore dermal bones. Now it is indisputable that these are the homotypes or serial 
homologues of the second to the eighth of the same median series of plates. The 
mere circumstance of connation of these plates with the closely subjacent vertebrse 
cannot make so essential a difference as is implied by their classification in a distinct 
skeleton-system from that to which their homotypes are admitted to belong. 
With regard to the ‘costal plates,’ M. Rathke, after rightly stating that “all, or 
nearly all, of the ribs of the trunk-vertebrae are cylindrical until the exclusion of the 
embryo,” proceeds to say, “ they then begin to increase in breadth ; this increase 
commences at the place where the neck of the rib joins the body of the rib, and 
thence advances more or less towards the (distal) extremity: it becomes so consider- 
able, that the bodies of all the ribs, by reason of the complete absence of intercostal 
muscles, come into contact on each side, either through their entire length, as in the 
genera Emys, Terrapene, Testudo, Trionyx, or nearly their whole length, as in Che- 
lone^." 
The author appears to have traced, with great industry and perseverance, the de- 
velopment of the carapace in each of the genera which he cites in the above quota- 
tion : but the very general terms in which such development is described might have 
been suggested by a mere comparison of its results as they are manifested in the adult 
carapaces, except that in no species of Trionyx are the ribs united throughout their 
entire length : the extremity of the actual rib projects from the peripheral end of the 
superincumbent costal plate, even in the oldest specimens. M. Rathke proceeds: — 
“ Soon after the eight pairs of ribs have begun to expand a process is sent off from 
above near the spinal column, which by its growth overlaps the few and slender 
dorsal muscles, and unites with the spinous process of the vertebrae -i-.” Such a de- 
scription of the development of the costal plates could be suggested, I believe, only 
by observation of a tolerably young specimen of Chelone or Emys. There is no 
mention of the development of the costal plates in the Tortoise {Testudo) by super- 
position of osseous matter upon the rib, the supporting part of which rib retains its 
normal form without expanding : there is no allusion to the alternately varying posi- 
tion of the superimposed dermal ossification in regard to the rib supporting it, nor to 
the relation of the incipient costal plates to the angles of union of the epidermal 
scutes. Perhaps these facts, so important in guiding us to the right homology of the 
costal plates of the carapace, were manifested in the young Tortoises examined by 
Prof. Rathke, though he has not described them : and yet it would be unfair, without 
* Loc. cit. Loc, cit. 
