347 



of the more recent authorities, beginning with W. K. Parker ( Structure 

 and development of the shoulder girdle and sternum in the vertebrata, 

 London, 1868), and Huxley (The Elements of Comparative Anatomy, Lon- 

 don, 1S64), consider the epiplastra and the entoplastron to be the homo- 

 logues of the clavicles and interclavicle respectively, of other reptiles. 



In form the entoplastron is quite as variable among the Testudiuata 

 generally, as are the paired elements associated with it. It is perhaps 

 most frequently T-shaped or roughly triangular, with the apex of the tri- 

 angle directed caudad. In Trionyx, however, it has an entirely different 

 configuration, being in the form of a wide V with the apex or point di- 

 rected cephalad (Fig. 1). 



The other elements of the plastron have outlines and relationships 

 characteristic of the family and can be easily identified by reference to the 

 figure (Fig. 1), wherein the epiplastra (epi) are shown immediately 

 cephalad of the entoplastron (ento), while the hyoplastra (hyo), hypo- 

 plastic (hypo), and xiphiplastra (xiph), lie caudad to that element in the 

 order given. 



In a Trionyx embryo with a carapace length of 14 mm., the elements 

 of the plastron are all definitely laid down (Fig. 2). The nuchal plate 

 of the carapace is a well marked and clearly defined dermal bone 

 having as yet no connection with a vertebra. The ribs are fully 

 laid down in cartilage, but there are no traces of costal plates, and 

 neurals, likewise, are not present. The plastral elements are not only 

 all present but they are also all paired. They are not preformed in carti- 

 lage but consist entirely of ossifications within the dermis. In shape and 

 size they are clearly defined. As shown in the figure (Fig. 2) they form a 

 series of five pairs of more or less rod-like structures, which are not in 

 contact with one another, as is the case in the adult (Fig. 1), but on the 

 contrary they are separated by comparatively large spaces in which the 

 tissue of the dermis is clearly mesenchymatous and shows no trace of ossi- 

 fication. The position of the five pairs in two longitudinal rows and their 

 absolutely similar origin as entirely dermal ossifications make it certain 

 that, whatever their homology to structures in other forms may be, they 

 must all be interpreted as serial homologues of each other. While it is 

 agreed that the hyoplastra, bypoplastra, and xiphiplastra are the homo- 

 logues of the abdominal ribs found in the Crocodile and Rhynchocephalia, 

 the epiplastra and entoplastron are pretty generally regarded as represent- 

 ing the clavicles and interclavicle of other reptiles. 



