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 Trionyx (Aspidonectes) spin if er, the plastron (Fig. 1) is com- 

 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 Knochen- und 

 Schalengeruestes, 182S), and Peters (Observations ad Anatomiam Chelo- 

 niorum. Berolini, 1S3S), 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, 1S49), advances 

 the idea that the paired plates correspond to haemapophyses of the ribs. 

 Rathke (Ueber die Entwickelung der Schildkroten. 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 



