SKULL 



317 



characterised by several special features. There are no ecto- 

 pterygoids or ossa transversa ; no lacrymal bones, no inter- 

 parietal or pineal foramen ; the vomer is unpaired and the 

 nasal bones are mostly absent, unless they are fused with the 

 prefrontals. The premaxillae are very small. The single vomer 

 forms a septum between the choanae ; and these are, except in 

 Spharyis, ventrally roofed over by wings sent out by the palatines. 

 The latter form a continuous bony roof to the mouth with the 

 pterygoids, and these diverge posteriorly, being connected suturally 

 with the quadrates, lateral and basi-occipital bones, and with 

 the unpaired basi-sphenoid, which appears between the basi- 



FIG. 63. Skull of Chelone mydas. 

 A, from the left side ; in B, the 

 postfrontal and squamosal bones 

 have been removed, and the broad 

 expansions of the jugal, quadrato- 

 jugal, parietal, and quadrate bones 

 have been reduced in order to re- 

 duce the skull to more primitive 

 conditions. F, Frontal ; J, jugal ; 

 L.o, lateral occipital ; MX, maxil- 

 lary ; Op, opisthotic ; Pal, palatine ; 

 /'"/, parietal ; /'/;/', prefrontal ; 

 Pro, pro-otic ; Pt.f, postfrontal ; 

 Ptg, pterygoid ; Q, quadrate ; Qj, 

 quadrato- jugal ; S.o, supra -occi- 

 pital ; Sr>, squamosal. 



occipital arid the diverging pterygoids, but is in most cases to a 

 great extent overlapped by the latter. The occipital condyle is 

 distinctly triple ; the basi-occipital sometimes helps to border the 

 foramen magnum. The supra-occipital sends out a long vertical 

 blade, directed backwards and generally projecting far over the 

 neck, for the attachment of the powerful cranio-cervical muscles. 

 The quadrate is very peculiar. Firmly attached, and hemmed 

 in on nearly all sides by the neighbouring bones, it stands nearly 

 vertically and forms a broad articulating surface for the mandible. 

 Its posterior side shows either a transverse, horizontal groove, in 

 which lies the columella atiris, or the groove is transformed into 



a more or less closed canal. Moreover, the hinder lateral maruin 







of the quadrate forms most of the tympanic frame ; its margins 

 being curved backwards, leaving in the Cryptodira, however, a 



