132 SKULL OF THE TORTOISE. 



of tlie atlas, or "odontoid," has coalesced witli its proper 

 neural arch, which develops two transverse and two long 

 posterior oblique processes, as in the cheljs. 



In the true or land tortoises the temporal depressions 

 are exposed, as in the box-tortoises and fresh-water ter- 

 rapenes: the head is proportionally -small, and can be 

 withdrawn beneath the protective roof of the carapace. 

 The skull is rounder and less depressed than in the terra- 

 penes : the frontals enter into the formation of the orbital 

 border. The tympanic hoop is notched behind, but the 

 columelliform stapes passes through a small foramen. The 

 palatine processes of the maxillaries are on a plane much 

 below that of the continuation of the basis cranii, formed 

 by the vomer and palatines. In most of the chelonia, the 

 nasal bone is connate with the prefrontal ; and, in all, the 

 tympanic pedicle is firmly wedged between the broad 

 appendage of the maxillary arch, formed by the malar, 

 26, and squamosal, 27, in front, and the mastoid, 8, behind. 

 The broad-headed terrapene {^podocnemys expansa) differs 

 from other fresh- water tortoises, and approaches the ma- 

 rine tortoises (turtles), by the vaulted bony roof arching 

 over the temporal depressions. This roof is chiefly formed 

 by the parietals, but differs from that in the turtles in 

 being completed laterally by a larger proportion of the 

 squamosal than of the postfrontal, which does not exceed 

 its relative size in other terrapenes. The present species 

 further differs from the marine turtles in the nonossifica- 

 tion of the vomer, and the consequent absence of a septum 

 in the posterior nostrils ; in the greater breadth of the 

 pterygoids, which send out a compressed rounded process 

 into the temporal depressions : the orbits also are much 

 smaller, and are bounded behind by orbital processes of 

 the postfrontal and malar bones : the mastoids and paroc- 



