ORDER TU 



ICHTHYOSAURIA 



167 



externally by the squamosal and postfrontal. The external nares are triangulär 

 and placed close to the orbits ; the nasals are extended in front, and the 

 premaxillae greatly produced. The superior border of the orbit is formed by 

 the postfrontal, and prefrontal, the latter a narrow, triangulär plate. In 

 front of the orbit, and separating it from the narial opening, is a triangulär 

 lachrymal. A long, slender, rod-like jugal bounds the orbit below, and a 

 curved oblique postorbital encloses it behind. Joining the postorbital and 

 jugal posteriorly is the quadrato-jugal, which forms the postero-lateral angle 

 of the skull, and together with the quadrate furnishes the connection for the 

 lower jaw. An irregularly triangulär or rectangular plate, the supratemporal 

 (or pr'osquamosal. St), is inserted between the quadrato-jugal and squamosal, 

 and behind the postorbital and postfrontal. The maxilla is an elongate 

 triangulär bone, inserted between the jugal, lachrymal, and premaxilla. 



The basioccipital (Fig. 263) is very strongly developed, and bears a stout 

 articular condyle. Above it and on either side of the foramen magnum are 

 the paired exoccipitals, capped by the b a 



supraoccipital. Adjoining the ex- 

 occipitals are the opisthotics, which 

 remain separate from the proötics. 

 The bones of the occipital and otic 



Fig. 263. 



Ichthyoscmrus acutirostris, Owen. Upper I-,ia.s ; 

 England. Skull, posterior aspect, 1/4. (bo, Basi- 

 occipital condyle ; exo, Exoccipital on either side of 

 foramen niagnnin ; j, Quadrato-jugal ; pa, Parietal ; 

 pt, Pterygoid ; '/u, Quadrate ; quj, Supratemporal, 

 missing on the right-hand side ; so, Supraoccipital ; 

 sq, Squamosal ; xt, Opisthotic), (after Owen). 



Ichthyosmtrus acutirostris, Owen. Upper l^ias : 

 England. A, Palatal aspect of skull, ß, Right 

 temporal region, Vr,- ich Internal nares ; pl, Vomer ; 

 xph, Basisphenoid, its rostrum dividing interptery- 

 goid vacuitv ; t, Palatine, not extending so far in 

 either direction as shown. Lettering otherwise as 

 in preceding figures), (after Seeley). 



regions are rather loosely united, and the base and lateral walls of the brain 

 case appear to have been cartilaginous ; there is no ossified alisphenoid or 

 orbitosphenoid. 



Abutting against the basioccipital on the lower side of the skull (Fig. 264) 

 is a more or less discoidal basisphenoid, which develops a long, splint-like 

 anterior process (presphenoid), extending along the middle of a large inter- 

 pterygoid vacuity. The pterygoids are long and relatively large-sized bones 

 joining the vomer and palatines in front, and the basisphenoid and quadrate 

 behind. There is a well-developed columella or epipterygoid, but no ecto- 

 pterygoid has been observed. The posterior nares occupy their primitive 

 Position on either side of the narrow yomers. A pair of stout, rib-like hyoid 

 bones is known to have been present. 



