148 



VERTEBRATE SKELETON 



There are one or two pairs of vacuities in the floor of the cranium 

 (tig. 157), one lateral to the pterygoids, the second, when present, 

 between the lateral angles of the pterygoids and the palatines. 

 Vomers are always visible and sometimes fused in the middle line and 

 also with the palatines. Except in Cistudo, Hydromedusa, Trionyx, 

 etc., the pterygoids meet in front (fig. 155) and are articulated with 

 the smaller basisphenoid by their medial margins, and. except in 

 Chelonid^, they reach the maxilla?. Where the pterygoids do not 

 meet, the basisphenoid extends to the vomer, except in Trionyx 



mx 



Fig. 157. — Cranium of Trionyx. bo. basioccipital; bs, basisphenoid; eo, es, exoc- 

 cipital;/, frontal; j, zygomatic; m. mx, maxilla; n, prefrontal; p, (behind orbit) post- 

 frontal; others parietal; pal, palatine; pmx, premaxilla; p7io, prootic; pt, pterygoid; q. 

 quadrate; s, supraoccipital; v, vomer. 



where the palatines intervene. A distinct presphenoid occurs in 

 Dermochelys and in the ontogeny of Chelys; but is not known in other 

 forms. An os transversum is unknown. 



The quadrate is expanded and excavate hke its cartilage, and in 

 the adult the columellar notch in its posterior border is sometimes 

 converted into a foramen by the meeting of the margins of the gap, 

 a condition met also in Pythonomorphs. The axis of the quadrate 

 is nearly vertical and the bone is held immobile by squamosal and 

 quadratojugal when the latter is present. 



