LIASSIC FORMATIONS. 103 



The interpterygoid vacuity (PI. XXV, fig. 1, *) is a very long and narrow triangular 

 one; its base is behind, and is bounded by the anterior concavities of the basisphenoid. 

 As it advances it is divided for a certain extent by the presphenoidal rostrum ; the lateral 

 boundaries are due exclusively to the pterygoids, which, converging, reduce the aperture 

 to a point. In this formation, Ichthyosaurus resembles Iguana, but in Varanus the 

 palatines enter into the formation of the palatal vacuity in question. 



The pterygomalar vacuity (PI. XXV, fig. 1, y ) may be described as the lower outlet 

 of the temporal fossa ; it is bounded laterally by the zygomatic and malar, chiefly the latter 

 bone; posteriorly by the tympanic, anteriorly by the maxillary, mesially by the ectopterygoid 

 and pterygoid ; the outer deep emargination of the latter bone relates to the passage of the 

 large temporal muscles for insertion into the mandible. The larger proportion which the 

 maxillary contributes to this vacuity in recent Saurians suggested the term ' pterygo- 

 maxillary,' applied to it in the undercited work. 1 



The palato-nares (ib., ^m) are relatively smaller than in most Lizards, and are circum- 

 scribed by the palatine, ectopterygoid, maxillary, and premaxillary bones. In Lizards the 

 ectopterygoid is excluded from its formation, and the position of the palato-naris is more 

 advanced than in Ichthyosaurus. The Crocodilia depart widely from the Ichthyosauri 

 in their single and backwardly placed palatal nostril. 



The 'foramen parietale' (PI. XXIII, fig. 1,/) receives a much ( smaller proportion, if 

 any, of the parietal bones than of the frontals in its formation (PL XXVI, fig. 1). 



The upper outlet of the temporal fossa (Pis. XXIII and XXVII, fig. 1, t) is bounded 

 mesially by the parietal, laterally by the mastoid and postfrontal, behind by the mastoid 

 and parietal, in front by the postfrontal ; the lateral or outer wall of the fossa is formed 

 by the mastoid, postfrontal, prosquamosal, postorbital, malar, and zygomatic bones. 



More or less of the circle of sclerotic plates are commonly preserved in the fossil 

 skulls of Ichthyosauri. They are of an irregular, oblong, quadrate form, joined together 

 by squamous or overlapping sutures at their longest sides. The hind part is usually 

 about half the length of the plate, and is very thin, ending in a trenchant border ; the 

 front or pupillary corneal border is thicker, shorter, and nearly straight. Prom this 

 border each plate extends, raylike, outward, for more than half its length, then suddenly 

 bends towards the back of the eyeball, defining and encasing its periphery, and indicating 

 the extreme oblateness of that visual spheroid. In the Ichthyosaurus communis I have 

 counted seventeen of these sclerotic plates. 



c. Pectoral and Pelvic Arches and Appendages. — The limbs of Ichthyopteryyia, as the 

 name of the order implies, resemble the fins of Pishes in the number of digital joints or 

 segments, and, in some species, the seeming excess of digits beyond the typical ' five.' 

 With the parial ones of Fishes these Reptilian fins also correspond, the anterior pair with 

 the ' pectorals,' the posterior pair with the ' ventrals.' 



1 'Anatomy of Vertebrates,' vol. i, p. 156. 



