156 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
understood Carus’s idea, by the figure of the skeleton of a young Sea-turtle {Chelone 
caoiianna), which he explains in his excellent ‘ leones Zootomicse,’ fol. 1841*, Tab. XIV. 
fig. 12 ; where a are the ribs, b the vertebral bodies, c the neural arches (bogentheile), 
d the neural spines, and ee the median row of dermal bones (‘mittlere Reihe der 
Hautknochen,’ p. 17). Now these latter, in the figure, are six in number, extending 
from one end of the carapace to the other, whilst the subjacent neural spines agree 
in number with the vertebrse, of which there are twelve between the scapula and 
ilium. It is plain, therefore, that the horny ‘vertebral scutes,’ as they are called in 
Erpetology, are here the parts supposed to represent the dermo-skeleton, and that 
the bony ‘ neural plates ’ are regarded as the spinous processes, agreeably with the 
Cuvierian view. 
Prof. Rathke'I' has recently propounded another modification of the combined 
dermo- and endo-skeletal hypothesis of Carus. Finding that there were no osseous 
plates developed independently in the corium and afterwards coalescing with the 
neural spines and ribs, as Carus and Wagner describe, he concludes that the cara- 
pace of the Chelonia is composed exclusively of endo-skeletal elements, but that the 
plastron as exclusively consists of exo-skeletal parts or dermal bones, in which cate- 
gory also he places the ‘marginal pieces,’ sufficiently proved by the Trionyx and 
Sphargis to be not essential to the composition of the carapace. 
The special deductions by Rathke will be compared, in the sequel, with my own 
observations on the development of the carapace in the Chelonia ; but it will be ob- 
vious, from the conflicting opinions on the nature and homologies of the chelonian 
skeleton, published within the last ten or fifteen years, that the question is far from 
having been satisfactorily settled ; and that no one can be regarded as giving the re- 
quisite description of the carapace and plastron who merely adopts the determina- 
tions of Geoffroy, or Carus or Rathke, without first testing them by an appeal to 
nature, and assigning the grounds of his acceptance, rejection or modification of such 
determinations. 
Commencing by the way of a comparison of the skeletons of fully-developed Ver- 
tebrata, and assuming for the purpose of such comparison that the thoracic-abdo- 
minal case is a modification of parts of the endo-skeleton, as Cuvier, Geoffroy and 
Meckel believed, I propose in the first plaee to test the homologies which have been 
generally accepted in this country, and of which, as regards the ‘ marginal plates,’ so 
positive an opinion has been recently published. 
Geoffroy St. Hilaire was guided, as is well known, to his conelusions by the 
* “ Hier ist das skelet einer jungen Seeschildkrote (fig. xii) zu vergleichen, wo man sieht, dass Wirbelsaule, 
Rippen und Brustbein in ihrer ursprunglichen Anlage von dem eigentlichen Riicken- und Briistschild ganz 
getrennt sind ; das dieses eigentlich aus isolirten Verknocherungen in der Haut entsteht, welche erst spater mit 
Knochenskelet verwachsen.” p. xii. 
t Sur le development des Cheloniens. Annales des Sciences, Mars, 1846 ; and Ueber die Entwickelung der 
Schiidkroten. 4to. 1848, p. 122. 
