346 
No other terrestrial or freshwater tortoises possess so simple and per- 
haps so primitive a type of plastron as that found in the Trionychia. In 
the adult Trionyxr (Aspidonectes) spinifer, the plastron (Fig. 1) is c¢om- 
posed of nine elements, four paired and one unpaired, separated to a 
greater or less extent at first by three, and later sometimes by only two, 
large fontanelles. Different authors have proposed different theories rela- 
tive to the homologies of these plastral bones, and along with these theories 
there has arisen a complex terminology. Each author has sought to give 
permanency to his own hypothesis by assigning to the plastral elements 
names indicative of his view. Thus the unpaired element is designated by 
G. St. Hilaire, Owen, Ruetimeyer, and others, who regard the plastron as 
the homologue of the amniote sternum, as the “ento-sternal’; Parker calls 
it the “inter-thoracic plate’; while Huxley gives it the noncommittal 
name of “ento-plastron,” in which he is followed by most later writers. 
The four paired elements of the plastron have not fared any better. Thus, 
G. St. Hilaire, Owen and Ruetimeyer designate them as “episternal,” 
“hyosternal,” “hyposternal,” and “xiphisternal,” respectively; Parker, as 
’ 
usual, has his own set of terms, and calls them “praethoracic,” “post- 
thoracic,” “praeabdominal,” and “abdominal” plates; while Huxley gives 
them the names of “epiplastron,” “hyoplastron.” “hypolastron,” and “xiphi- 
plastron.” In the present state of our knowledge it is best, perhaps, to 
use Huxley’s terms, since they commit one to no special theory regarding 
the homologies of the elements to which they apply. 
Among the various attempts that have been made to homologize the 
plastral plates with certain skeletal elements of other amniotes, one of 
the earliest was that of Cuvier (Regne animal, Les Reptiles, p. 10), who 
identifies them with the sternum of the Lacertilia and higher vertebrates. 
G. St. Hilaire (Philosophie anatomique, vol. i. p. 106) makes a detailed 
comparison between the several parts of the plastron and the osseous 
pieces of the avian sternum. Carus (Von den Ur-Teilen des IKnochen- und 
Schalengeruestes, 1S28), and Peters (Observationes ad Anatomiam Chelo- 
niorum, Berolini, 1888), maintain that it is only partially equivalent to the 
sternum. Owen (On the development and homologies of the carapace and 
plastron of the Chelonian Reptiles, Phil. Trans. London, 1849), advances 
the idea that the paired plates correspond to haemapophyses of the ribs. 
Rathke (Ueber die Entwickelung der Schildkréten, Braunschweig, 1848), 
holds the plastron to be wholly dermal in origin and hence a structure not 
to be homologized with the endoskeletal elements of other groups. Many 
