164 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
of the segment without the intervention or aid of any of the marginal plates, the 
seventh of these being simply applied to the outside of the hyposternal, where its 
slender elongated extremity bends upwards to join the vertebral rib : and the only 
incomplete part of the arch is the unossified median space between the lower ex- 
panded and dentated ends of the hyposternals, between which the entosternal, or true 
sternal piece, does not extend backwards. So that the condition of this fifth segment 
of the thoracic-abdominal box, in the young Tortoise, repeats that of a posterior dor- 
sal segment of a mammal or crocodile, in which the cartilages of the ribs, or abdo- 
minal ribs, do not reach the sternum ; and the Ornithorhynchus offers a special re- 
semblance to the Tortoise in the expansion of the semiossified hsemapophyses, or 
cartilages of its ‘false ribs.’ The xiphisternals, viewed in like manner as ‘hsem- 
apophyses’, repeat the condition of those abdominal ones in the Crocodile and Plesio- 
saur which do not ascend so high as to join their pleurapophyses or vertebral ribs. 
The difference between the endo-skeletal and exo-skeletal portions of these elements 
of the plastron is as plain, and the contrast, indeed, is almost as great, in the young 
Tortoise as in the adult Trionyx, where the superadded ossification, at the expense 
of the dermal system, is characterized by the vermicular or punctate character of the 
exterior surface, a character common to the dermal ossified plates in the Reptilia, 
especially of the closely-allied Crocodilian order . 
The main purpose of the augmentation of the ordinary vertebral elements in the 
thoracic-abdominal region of the Chelonia, by the extension of ossification from them 
into the corium, and the consequent connation with those elements of dermal bony 
* The distinction between the exo-skeletal and endo-skeletal parts of the plastron is so well-marked in the 
Trionyx, that the true explanation of the structure has forced itself, as it were, upon the authors who have 
given the most unqualified adhesion to the Cuvierian and GeofFroyian hypothesis. “ II est plus important de 
rappeler ici les caracteres principaux — qui distinguent I’ordre des Tortues des trois groupes d’animaux ranges 
dans cette meme classe des Reptiles ; d’abord de tons les autres genres par la structure de leur squelette, dont 
les pieces qui constituent le tronc sont exterieures. Les vertebres du dos, des lomhes et du bassin etant sou- 
dees et solidement articulees, non seulement entre elles, mais avec les cotes et quelquefois le sternum, par de 
veritables sutures, ou unies par cette sorte d’engrenage que I’on nomme synarthrose •, le tout forme ainsi une 
sorte de boite, — une ‘carapace’ !— La partie inferieure du corps est egalement protegee par des pibces osseuses, 
correspondantes a un sternum, dont I’ensemble porte le nom de ‘plastron.’ ” — Dumeril and Bibron, Erpeto- 
logie Generate, 8vo, tom. i. p. 349, 1834. 
The description of the carapace of a species of Trionyx is as follows : — “ Cette esp^ce et la suivante sont les 
seules ou I’on ne compte sur le disque de la carapace que sept callosities costales de chaque cote d’ I’epine dor- 
sale, encore que ces deux especes aient reellement huit paires de cotes comme tous les autres Gymnopodes. 
Cela vient de ce que chez le Gymnopode spinif^re et chez le Gymnopode mutique il n’existe qu’une seule cai- 
losite pour les deux dernieres cotes de chaque cote, tandis que dans les autres Gymnopodes les seize prolonge- 
mens costaux ont chacun leur callosite.” The part here denominated ‘callosity’ is the connate dermal bone 
which is described in this memoir as the ‘ costal plate ’ ; but it is not more distinct in its mode of development, 
nor less connate with the subjacent rib in those Trionyces, which MM. Dumeril and Bibron call ‘Gymnopodes,’ 
than it is in the ordinary Tortoises, Terrapenes and Turtles : only the superficial character of the superadded 
part is more distinct in the Trionyces : but it failed to draw the attention of the distinguished French erpeto- 
logists to a reconsideration of the homologies of the carapace which they had adopted. 
