CARETTOCHELYDIDAE TRIONYCHOIDEA 



405 



tion of being arched or l3ridged over. The premaxilla is extremely 

 small, unpaired, not even reaching the nasal cavity or the 

 vomer. The maxillaries are 

 correspondingly enlarged, 

 surrounding the choanae, 

 which are separated by the 

 narrow vomer. The palatines 

 form a median suture, and 

 are joined behind by the long 

 basisphenoid, which separates 

 the long pterygoids from 

 each other. The quadrate 

 is trumpet -shaped, with a 

 posterior notch for the stapes. 

 The zygomatic arch is com- 

 plete, and is formed by the 

 quadrato-jugal and the jugafl; 

 the latter joins the maxillary 

 and postfrontal, mostly reach- 

 ing the orbit ; in some cases 

 it also just meets the parie- 

 tal, thereby adding to the 



strength of the postorbital Fi«- 91.— Skull of Trionyx hurum. A, From 



. - above ; B, from the left side ; Cond, occij)ital 



arch. The preirontals are condyle ;/-/-, frontal ; J^, /;<//, jugal ; Z.o, 



large ; nasals are absent. The lateral occipital ; Max maxillary ; Op.o, 



. . ^ o opistliotic ; Par, parietal ; Pr.f, prefrontal ; 



mandible is remarkable for pw, prootic ; Pt.f, postfrontal ; Q, qnad- 

 the great develoinnent of """tV %' q^a'^^^^-J^g^l ; '^^•^' ^upra-occi- 



^ _ i pital ; Sq, squamosal. 



the coronoid process. 



The pubic and ischiadic bones enclose a large heart-shaped 

 foramen, and are free from the plastron ; the ilia are attached 

 only to the sacral ribs. The carapace is peculiar in so far as it 

 is very incomplete peripherally, the ribs extending considerably 

 beyond the costal plates, nor are they joined by marginal plates, 

 which are absent, unless they are represented by a few small 

 ossifications imbedded in the posterior marginal flap of the disc 

 {Emyda of India). The rim of the disc is always formed by a 

 horizontal, cutaneous, very flexible flap. All the dorsal plates 

 have a rough upper surface, vermiculated or rugose, as usual 

 with such dermal bones, which have lost most of or all their 

 horny covering, and have sunk more deeply into the skin. The 



