OR OSSEOUS SYSTEM. 83 



ribs. The two epi-sternal pieces (a, ,) form the anterior 

 margin, and unite behind with the ento-ster- FIG 

 nal piece (,) which is single on the median 

 plane, and with the two hyo-sternal pieces 

 (e, c,) which extend laterally to unite with the 

 sternal ribs. The two hyposternal elements 

 (d, d,) are large and broad, like the hyo-ster- 

 nal, and also like them unite laterally 

 with the sternal ribs. The two xiphi-sternal 

 pieces (e, e,) are united to each other, and to 

 the hypo-sternals by serrated sutures, and form 

 the whole posterior termination of the plastron 

 or lower shield. In the sternum of the tur- 

 tles (Fig. 43, 2.) nearly all these elements are moveable on 

 each other, and at their points of contact with the sternal 

 ribs. The two epi-sternals (, #,) taper laterally to a point, 

 and are not united by sutures ; the ento-sternal (#,) has its 

 whole posterior portion free, and although the hyo- (c, c,) 

 and hypo- (d, d,) sternals on each side are united together 

 by firm sutures, they have moveable articulations on their 

 lateral margins. The xiphi-sternals (e, e,) are here freely 

 moveable, and taper to a point posteriorly, like the two 

 epi-sternals. So that there is great solidity in the whole 

 plastron, as in the whole carapace of the land animals to 

 resist pressure, to which they are much exposed, and they 

 form a dense frame-work for their muscular movements on 

 the land, while there is great mobility in the sternal apparatus 

 of the aquatic species, for the extensive respiration which 

 they require in that dense element. 



The bones of the head in the chelonia, like those of the 

 crocodiles, are immoveably united by sutures, and in place 

 of teeth there are strong cutting, horny plates covering the 

 alveolar surface of the jaws, as in birds. The occipital 

 condyle is composed of three distinct facets, formed by the 

 basilar and the two condyloid portions of that bone. The 

 superior median portion of the occipital extends backwards 

 in form of a long spinous process, as in most osseous 

 fishes, for the attachment of the powerful muscles of the 

 neck. The two parietal bones are separate, and form in 

 the tortoises of the land an elevated longitudinal ridge, 

 which is continued forwards over the cranium to the frontal 



