122 
FOSSIL REPTILIA OF THE 
correspondiiig part of the right one above described, and so far confirms the inference as 
to the specific character of tlie fins associated with the acuminate one of the skull. 
The fossils on which the above species of Ichthyosaurs is founded are from the Lias 
of Whitby, Yorkshire. 
i. Ichthyosaurus tenuikostrts, Ch. Plate XXIV, fig. 8 ; Plate XXV, fig. 3 ; Plate 
XXXII, figs. 1 — 6 . 
The characters which, in 1822 ,^ strikingly distinguished the present from the then 
determined species of Ichiht/osaiirus, were the great length and slenderness of the jaw- 
bones, suggesting the pro])ortions of those of the Crocodilian gharrial ; and which, in 
combination with the large orbits and low broad cranium, gave to the skull a resemblance 
to that of a gigantic woodcock {Scolojjax), with a bill armed with teeth. 
The length of the snout is chiefly due to the prolongation of the premaxillaries 
(PI. XXXII, fig. 2 , 22) and dentaries (ib., 33). The length of the skull anterior to the 
orbit is somewhat less than four times the antero-posterior diauieter of that cavity. 
The parietals retain their sagittal suture, the fore part of which recedes in a greater 
proportion than usual to contribute to the foramen parietale. Their posterior bifurcation 
is applied, in the occipital region, to the super-occipital, which is broad and arched. The 
outer end of each parietal prong is obliquely truncate for the suture with the mastoid 
(ib., fig. 2 ). This, as usual, forms the blunt but prominent supero-lateral angles 
of the hind part of the cranium. The midfrontals (ib., 11) retain their suture, their 
lateral border articulates with the prefrontal (14) and the superorbital, or a forward 
extension of the postfrontal. This element articulates with the lateral border of the 
parietal and combines with that bone in forming the upper three fourths of the, temporal 
fossa, the lower boundary of that cavity being completed by the mastoid. 
The orbit is bounded behind chiefly by the postorbital. The malar is unusually 
long, extends from the lacrymal (73) to form the rest of the anterior boundary of the 
orbit ; then, continuing to circumscribe it below, the malar curves with a rather abi’upt 
bend upwards to join the postorbital, and by an oblique suture the zygomatic. This 
bone is continued obliquely backward to contribute to the external meatus auditorius and 
to articulate with the tympanic. 
The sclerotic plates preserved in the orbit of the subject of fig. 2 bend more 
abruptly than usual towards the back part of the cavity, suggesting a depressed spheroid 
form of the eyeball. 
The nasal (15) presents the usual connections; the margin which it contributes to 
the upper part of the nostril {n) is slightly convex, encroaching on that opening, which 
^ ‘Trans, of the Geological Society,’ 2nd series, vol. i. 
