LIASSTC 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 rescnnbles Iguana, but in Varanus the 
palatines enter into the formation of the palatal vacuity in question. 
The pterygonifflar vacuity (PL 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 pteiygoid ; 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.^ 
The palato-nares (ib., pv) 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 (PI. 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 ])ostfrontal, 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 Ichthyopterygia, 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 Pishes these Reptilian fins also correspond, the anterior pair with 
the ‘ pectorals,’ the posterior pair with the ‘ ventrals.’ 
^ ‘Anatomy of Vertebrates,’ vol. i, p. 156, 
