THE SKELETON OF THE TURTLE. THE PLASTRON. 217 



number, eleven lying on each side, while an unpaired one lies 

 in the middle line posteriorly. Many of them are marked by 

 slight depressions into which the ends of the ribs fit ; 



(e) the pygal plates (fig. 36, 5) are two unpaired plates 

 lying immediately posterior to the last neural. 



The sculpturing due to the epidermal shields is very 

 obvious on the carapace. 



The plastron (fig. 37) consists of one unpaired ossification, 

 the entoplastron, and four pairs of ossifications called re- 

 spectively the epiplastra, hyoplastra, hypoplastra, and 

 xiphiplastra. 



The epiplastra (fig. 37, 1) are the most anterior, they are 

 expanded and united to one another in the middle line in 

 front, while behind each tapers to a point which lies external 

 to a process projecting forwards from the hyoplastron. They 

 are homologous with the clavicles of other vertebrates. 



The entoplastron or episternum (fig. 37, 2) which is 

 homologous with the interclavicle of other reptiles, is ex- 

 panded at its anterior end and attached to the symphysis 

 of the epiplastra, while behind it tapers to a point and ends 

 freely. 



The hyoplastra are large irregular bones each closely 

 united posteriorly with the corresponding hypoplastron, and 

 drawn out anteriorly into a process which lies internal to that 

 projecting backwards from the epiplastron. Each gives off on 

 its inner surface a slender process which nearly meets its fellow, 

 while the anterior half of the outer surface is drawn out into 

 several diverging processes. 



The hypoplastra (fig. 37, 4) are flattened bones re- 

 sembling the hyoplastra, with which they are united by long 

 sutures ; the posterior half of both outer and inner surfaces 

 is drawn out into a number of pointed processes. 



The xiphiplastra are small flattened elongated bones 

 meeting one another in the middle line posteriorly. In front 

 they are notched and each interlocks with a process from the 



