ICHTHYOSAURIA 4/9 



is formed mainly by the premaxillary bones. The nostrils lie 

 far back, in front of the orbits, and are bordered by the long 

 nasals, the premaxillaries, a small part of the maxillaries, and 

 posteriorly by the large lacrymal bones. The eyes are large, and 

 are strengthened by a sclerotic ring composed of many closely over- 

 lapping bones. The orbits are very large, and are directed side- 

 ways so as to be scarcely visible from above. They are formed above 

 by the long prefrontals, which join the postfrontals ; behind by 

 the long postorbitals ; below by the long and slender jugals ; in 

 front by the lacrymals and prefrontals. The postorbito-temporal 

 region of the skull is short but high, and, with the exception of 

 the supratemporal foramen, is entirely closed in by bones, namely, 

 the quadrato -jugals, supratemporals, and squamosals. The 

 latter, with the parietals and large postfrontals, surround the 

 supratemporal foramina. The parietals and the small frontals 

 enclose the parietal foramen. The whole temporal arch conse- 

 quently recalls much that of the Pareiasauri and Stegocephali, 

 chiefly owing to the presence of conspicuous supratemporal and 

 postorbital bones, which, together with the quadrato-jugal, close 

 in the whole side without any indication of a lateral or infra- 

 temporal foramen. The postorbital completely separates the 

 jugal from the quadrato-jugal, and this almost hides the quadrate. 

 The occipital condyle is single. The lateral occipitals and the 

 supra-occipital bones retain their sutures. The pro-otic and 

 opisthotic bones remain separate. The latter lie between the 

 basi- and lateral occipitals, the squamosal, quadrate, and pterygoid. 

 The pterygoids, which posteriorly touch the quadrato-jugals, 

 basi-occipitals, opisthotics, and basisphenoid, are very long and 

 remain widely separated from each other ; in the space between 

 them appears the long ensiform presphenoid. Anteriorly they 

 are connected through the ectopterygoids with the maxillae, and 

 touch the palatines. These are likewise narrow and slender, but 

 touch each other in the middle line, and contain the well- 

 separated, slit -like choanae, laterally to which lie the elon- 

 gated, rather narrow, palatal vacuities. The vomers are mostly 

 not visible ; when they appear on the surface they are long 

 and narrow, and enclose the choanae between them and the 

 palatines. 



The teeth are pointed, conical and thickly covered with enamel, 

 which in transverse sections forms vertical ridges, recalling 



