SKELETON 



97 



participation of the squamosal in the hinge of the jaw. Lacrimals are occasion- 

 ally absent, sclerotics sometimes present. The palatal region is known in a few 

 forms. The pterygoids may meet only in front, leaving a vacuity between them 



Fig. 99. — Skull of Procohphon, after Woodward, bs, basioccipital; ep, epiptery- 

 goid;/, frontal; y, zygomatic (jugal); /, lacrimal; tnd, mandible; mx, maxillary; n, nasal. 

 pf, postfrontal; pi, palatine; pm, premaxillary; poo, postorbital; prf, prefrontal; g, 

 quadrate; qj, quadratojugal; spt, supratemporal; sq, squamosal; v, vomer. 



and the basisphenoid, or they may meet that bone. The choanae are in front of 

 the palatines but (theriodonts) may be displaced backward by palatine processes 

 of the maxillaries. 



Fig. 100. — Skull of Plesiosaurus tnacrocephalus, after Andrews, ang, angulare; 

 art, articulare; bo, basioccipital; bs, basisphenoid; ch, choana; d, dentary;/r, frontal* 

 i^<c, interpterygoid vacuity; j, zygomatic (jugal); wx, maxillary; ori, orbit; /'o, parietal; 

 Pal, palatme; pas, parasphenoid ; ^0 + pof, postorbital and postfrontal: pmx, premaxil- 

 lary; prf, prefrontal; pi, pterygoid; q, quadrate; sang, surangulare; sf, supratemporal 

 fossa; sov, suboptic vacuity; sq, squamosal; ir, transversum. 



All four occipitalia are developed; the occipital condyle is tripartite, being 

 formed by basi- and exoccipitals, but in Cynognathus the recession of the basi- 

 occipital results in a dicondylic condition. The greatest variations occur in the 



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