LIASSIC ICHTIIYOSAURS. 51 



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 (Tab. XXII, 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 (Tab. XXI, 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 

 (Tab. XXII, fig. 1, 2, 2) are small andreniform ; 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 (Tab. XXI, fig. 1,5) 

 laterally with the paroccipitals (Tab. XXII, fig. 1,4); but between these and the basisphe- 

 noid it joins the mesially inclined hinder end of the pterygoids (ib., 24). The apex of 

 the superoccipital (3) is wedged into the interspace of the hinder bifurcation of the parietal 

 bones ("'), which it underlaps and partly supports ; its base forms the upper border of the 

 foramen magnum. The paroccipitals (ib., 4, 4), broadest where they join the basioccipital, 

 contract as they extend outward into a strong triedral bar, which abuts against the 

 tympanic (28), at the interval between the mastoid (8) and pterygoid (24). 



The centrum (l), neurapophyses (2), neural spine (3), and parapophyses (4) of the 

 hindmost cranial vertebra are instructively demonstrated by the Ichthyosaurian condition 

 of the ' occipital bone ' of Anthropotomy. 



The basisphenoid (Tab. XXI, fig. 1, 5) presents, on its under and outer surface, the 

 form of an irregular, subquadrate plate, narrow^est behind, where it joins the basioccipital, 

 expanding as it advances, the anterior border presenting a rough, sutural, notched 



