CHELONIA. 227 



to 9thA appear in the median line as horizontal plates [neural 

 plates], the ribs of the same vertebrae (2nd to 9th, these ribs are dis- 

 tinguished from the first and last ribs by their greater length) are 

 transformed into broad transverse plates [costal plates], which are 

 joined with one another by indented sutures, and present the special 

 peculiarity of giving off broad processes, which arch over the muscles 

 of the back, and are connected with the neural plates (expanded 

 spinous processes). In addition, larger plates, which owe their 

 origin to cornifications of the epidermis, are usually present. They 

 are applied to the outer surface of both the dorsal and ventral shields 

 and are used, in the case of some of the larger species, as tortoise- 

 shell. They by no means correspond with the subjacent bony pieces, 

 but are very regularly arranged in such a manner, that in the dorsal 

 shield a median and two lateral rows of plates can be distinguished, 

 and round the periphery a circle of marginal plates. On the ventral 

 surface, on the other hand, there is a double row of such plates. 



Unlike the middle (thoracic) region of the vertebral column, the 

 vertebrae of which are firmly connected with the dorsal shield, the 

 cervical and caudal vertebrae are always movable upon one another. 

 The cervical region is exceedingly flexible, and can be more or less 

 completely retracted within the shell ; it consists of eight long ver- 

 tebrae, which are without ribs. The ten rib-bearing vertebrae are 

 followed by two or three sacral vertebrae, which project beneath the 

 carapace, and by a considerable number of very movable caudal 

 vertebrae. 



The head is tolerably arched : the bones of the skull are firmly 

 united to one another by sutures, and form a broad roof, which is 

 prolonged into a strongly developed occipital crest. The skull is 

 characterised by the possession of a pair of parietal bones and of 

 large anterior f rontals. Descending lamellar processes of the parietal 

 bones extend along the sides of the cartilaginous cranial capsule as far 

 as the short basisphenoid. The temporal fossa is most completely 

 roofed in in the marine Chelania by broad osseous plates which are 

 formed by the postfrontal, jugal, quadrato-jugal, and the squamosal. 

 The opisthotic remains as an independent bone behind the prootic, 

 which forms the lateral walls of the cranial cavity. All the parts of 

 the maxillo-palatine apparatus as well as the quadrate are firmly 

 connected with the bones of the skull, and are marked off from one 

 another by serrated sutures. The facial parts of the skull are 

 strikingly short, and the nasal bones are absent. The bony palate 

 is formed by the broad palatine and the unpaired vomer, behind th e 



