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, 1864), consider the epiplastra and the entoplastron to be the homo- 
logues of the clavicles and inuterclavicle respectively, of other reptiles. 
In form the entoplastron is quite as variable among the Testudinata 
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- 
plastra (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, hypoplastra, 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. 
