688 



PHYLUM CHORDATA : CLASS REPTILIA 



Some Peculiarities in the Skeleton of Chelonia 



The (lo) dorsal vertebra? are without transverse or articular processes, 

 and along with the ribs are for the most part immovably fused in the 

 carapace. The tail and neck are the only flexible regions. There are 

 two sacral vertebrae. 



The greater part of the dorsal shield is due to a coalescence of eight 

 ribs with eight costal plates derived from the dermis. 



Similarly, the median pieces are the result of fusion between median 

 dermal bones and the neural spines of the vertebra?. The plastron 

 usually consists of nine dermal bones, and the three anterior pieces 



perhaps represent clavicles and inter- 

 clavicle (or episternum). 



The eight cervical vertebra? have at 

 most little rudiments of ribs, are remark- 

 ably varied as regards their articular 

 faces, and give the neck many possibili- 

 ties of motion. There are no lumbar 

 vertebrae. 



The bones of the skull are immovably 

 united ; there is only a lower temporal 

 arcade, formed b}^ jugal and quadrato- 

 jugal ; there are no ossified alisphenoids, 

 but downward prolongations of the large 

 parietals take their place ; neither pre- 

 sphenoid nor orbitosphenoids are ossi- 

 fied ; there are no distinct nasal bones 

 in modern Chelonians, their place being 

 tjfken by the prefrontals ; the pre- 

 maxillae are verv small ; there are no 

 teeth. 



There is no sternum. The pectoral 



girdle on each side consists of a ventral 



coracoid and a dorsal scapula attached 



to the carapace. The scapula bears an 



anterior process of large size, usually regarded as a " precoracoid " or 



procoracoid. 



■ The pelvic girdle consists of dorsal ilia attached to the carapace, 

 posterior ischia, and anterior pubes, with pre-pubic processes and an 

 epi-pubic cartilage. There is a pubic and an ischiac symphysis. 



The girdles originally lie in front of, or behind the ribs, but are over- 

 arched by the carapace in the course of its development. 



Fig. 



399. — Carapace of 

 tortoise. 



The dark contours are those of 

 the bony pieces ; the lighter 

 contours are those of the scales 

 which have been removed. 



Some Peculiarities in the Organs of Chelonia 



In Chelonians and in all higher animals except serpents, there are 

 twelve cranial nerves, for, in addition to the usual ten, a spinal 

 accessory to cervical muscles, and a hypoglossal to the tongue, are 

 ranked as the eleventh and twelfth. 



The gullet of the turtle shows in great development what is hinted at 



