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XI. On the Development and Homologies of the Carapace and Plastron of the Chelo- 
nian Reptiles. By Professor Owen, F.R.S. ^c. 
Received November 16, 1848, — Read January 18, 1849. 
Those animals to which, in the manifold modifications of the organic framework, a 
portable dwelling or place of refuge has been given, in compensation of inferior powers 
of locomotion or other means of escape or defence, have always attracted especial 
attention ; and of them the most remarkable, both for the complex construction 
of their abode as well as for their comparatively high organization, are the reptiles 
of the Chelonian order. The expanded thoracic -abdominal case, into which, in most 
Chelonians, the head, the tail and the four extremities can be withdrawn, and in some 
of the species, be there shut up by moveable doors closely fitting both the anterior 
and posterior apertures, as e. g. in the Box-tortoises {Cinosternon, Cistudo), has been 
the subject of many and excellent investigations ; and not the least interesting result 
has been the discovery, that this seemingly special and anomalous superaddition to 
the ordinary vertebrate structure is due, in a great degree, to the modification of 
form and size, and, in a less degree, to a change of relative position, of ordinary 
elements of the vertebrate skeleton. 
To ascertain the precise nature and extent of these modifications, in other words, 
to determine the homologies of the bony framework of the case in question, is the 
aim of the present communication. 
The natural dwelling-chamber of the Chelonia 
consists chiefly, and in the marine species {Che- 
lone) and mud-turtles {Trionyx) solely, of the 
floor and the roof : side-walls of variable ex- 
tent are added in the freshwater species {Emy- 
dians) and land-tortoises {Testudinians). The 
whole consists chiefly of osseous ‘ plates ’ with 
superincumbent horny plates or ‘ scutes,’ except 
in the soft or mud-tortoises {Trionyx and Sphar- 
gis), in which these latter are wanting. It is 
requisite briefly to allude to the well-known 
composition of the osseous framework of this 
chamber in order to define the terms by which 
certain parts will be adverted to in the course 
of the paper. 
The roof or ‘carapace’ (fig. 1) consists of a 
Fig. 1. 
Carapace of the Loggerhead Turtle {Chelo7ie caouaiinu). 
