LIASSIC FORMATIONS. 
93 
The general form of the skull of the typical species. Ichthyosaurus communis 
(PI. XXVIII, fig. I), as in Ich. breviceps, Ich. mtermedms , Ich. lonchiodon, and Ich. 
jjlcdyodon} resembles that of the Cetaceous Dolphins {Delphinus tursio and Delphinus 
delphis). In Ich. acutirostris (ib., fig. 2) the beak is produced to the shape of that of a 
gigantic stork, while Ichthyosatirus tenuirostris and Ich. lonyirostris (ib., fig. 3) rival or 
surpass the Belph.inus {IniaJ gangeticus in the length and slenderness of the jaws. 
'I’he main difference in the Sea-reptiles lies in the restricted capacity of the brain-case, 
the seeming expanse of the cranium being due to the great depth and breadth of the post- 
orbital part of the zygomatic arches or outer walls of the temporal fossae, A more essential 
departure from the warm-blooded Vertebrates is the persistent individuality of those 
cranial elements which, though primitively distinct, become blended into single bones in 
the higher and later developed forms. The Ichthyosaurs further differ from the 
marine mammals in the great extent of the premaxillary and the small size of the maxillary 
bones, in the great capacity of the orbits and the circle of sclerotic plates lodged therein, 
and, finally, in the antorbital vacuities serving as external bony nostrils. 
The occipital region of the skull (PI. XXVI, fig. 1) is of great breadth and of mode- 
rate height. In its formation there enter not only the basi-, ex-, super-, and par-occipitals, 
but also part of the parietals, mastoids, tympanies, zygomatics, prosquamosals, and 
pterygoids. 
The chief feature is the large proportional size of the basi-occipital (ib., i), the outer 
surface of which is divided into an articular (PI. XXV, fig. 1, 1') and non-articular part 
(ib., 1). The articular portion is in the form of a hemispheric, convex condyle, in some species 
showing a subcentral depression, but deriving no contribution from the exoccipitals, and 
divided from the ‘ foramen magnum ’ by a narrow, upper non-articular tract ; the lateral 
tract gains breadth as it descends along the sides of the condyle, below which it shows an 
extent of two thirds the diameter of the condyle ; but this part of the basioccipital, in 
extending forward, deviates little from the perpendicular, and belongs rather to the 
hinder than the under surface of the cranium. The upper non-articular part of the basi- 
occipital, dividing, in the specimen under description, the condyle from the foramen 
magnum, is one eighth the diameter of the condyle ; then come the depressions for the 
sutural joints with the exoccipitals, a mere crest dividing them. The exoccipitals 
(PI. XXVI, fig. 1, 2, 2 ) are small and reniform ; their bases almost meet above the basi- 
occipital; their obtuse summits are divided by the base of the super-occipital ( 3 ), which 
contributes about a fourth part of the circumference of the foramen magnum (/). 
The fore part of the basioccipital presents, in some species, a slight notch or groove, 
as if for the outlet of an Eustachian canal. This canal, in Crocodiles, traverses the basi- 
sphenoid, close to its suture with the basioccipital. 
The basioccipital articulates, below or in front, with the basisphenoid (PI. XXV, fig. I, 5 ) 
^ See Clift’s excellent figure in ‘Philos. Trans.,’ mdcccxiv, pi. xvii. 
