134 ANATOMY OF VERTEBRATES. 



limits of the Chclonian order. Figs. 90 and 91 exemplify that 

 which prevails in the marine species (Chelone). In them the head 

 is proportionally larger ; and, being incapable of retraction within 

 the carapace, is additionally protected by extension of bone into 

 the fascia covering the temporal mnscles, so as to form a complete 

 ossens vaulted roof over the temporal fossae, due to exogenous 

 growths from the postfrontals, fig. 91, 12, the parietals, 7, and the 

 mastoids, 8. 



In the almost sole instance in which such accessory defence is 

 afforded to a non-marine species — the Brazilian Pipitu (Podo- 

 cnemis expansa) — the tempoi-al roof is chiefly formed by the 

 parietals, and is completed laterally by a larger proportion of the 

 squamosal than of the postfrontal, wdiich does not exceed its 

 relative size in other Terrapenes. The present species further 

 differs from the marine Turtles in the non-ossification 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 paroccipitals are more produced backward, and the entire 

 skull is more depressed than in the Turtles. In other freshwater 

 Tortoises (Emys, &c), the parietal crista is continued into the 

 occipital one without being extended over the temporal fossa? ; 

 the fascia coverinsr the muscular masses in these fossa; undergoing 

 no ossification. The bony hoop for the membrana tympani is 

 incomplete behind, and the columelliform stapes passes through a 

 notch instead of a foramen to attain the tympanic membrane. 

 The mastoid is excavated to form a tympanic air-cell. 



In the true Tortoises the temporal depressions are exposed, as in 

 the Terrapenes : 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 withdrawn beneath the protective roof of the carapace.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 Terrapenes. 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 the soft-turtles {Trionicidca), the skull is long, depressed, 

 triangular, the muzzle forming the obtuse apex, and the base 

 remarkable for its four backward prolongations. The inferior of 

 these is the shortest, and terminates in the occipital condyle ; the 

 superior is the longest, and is formed by the superoccipital spine : 

 the two lateral processes are developed from the paroccipitals and 



