LIASSIC FORMATIONS. 
99 
Crocodiles and Lizards ; in these, it is limited to a small part of the dentary element, 
but in Iclithjosaurus it includes both dentaries and splenials, and the ‘ symphysis dentarii ’ 
(PI. XXV, fig. 2, 33) is to be noted together with the ' symphysis splenii ’ (ib. ib., 32') in 
the discrimination of species. Thns, the latter is longer than the dentary symphysis in Ick. 
intermedins, but is shorter in Ich. communis, and, relatively, much shorter in Icli. lenui- 
rostris (PI. XXV, fig. 3 ), Ich. longirostris (ib., fig. 2 ), and other long and slender-jawed 
kinds. 
The vomerine bones are narrow, longitudinal, vertically disposed plates, with their 
lower margins rarely visible at the interspace of the palatines and pterygoids. 
The prefrontal (PI. XXIV, fig. 1, 14) I, as yet, know only by its external or facial 
part. This is a narrow, moderately long, bony tract, extending from the postfrontal to 
near the nostril (»*), there receiving the upper angle of the lacrymal (73) in a notch, the 
upper branch of which notch is wedged between the lacrymal and nasal, (15) ; the lower 
boundary is bent inwards to form part of that of the orbit. The extensive upper and 
inner border of the prefrontal articulates with the nasal, the short hinder border with the 
postfrontal. 
The position and relations of the prefrontal in Ichthyosaurus, as in some Fishes, 
instructively illustrate its general homology as an element of a cranial segment distinct 
from that to which the frontal belongs, of which bone it has been regarded as a mere 
dismemberment. As the neurapophyses of the nasal segment they lend, in Ichthyosaurus, 
a large share of their longitudinal extent to the support of their neural spine, the nasal 
bone. The large size of both pre- and postfrontals relates to that of the eye and of the 
cavity destined to contain it in the Ichthyosaurus. 
The nasals (Pis. XXIII, XXVII, figs. 1,15) are the longest and largest bones of the 
cranium proper, but contribute only a small part to the side face ; each sends a pointed 
process backward into a corresponding notch of the frontal which it partially overlaps. The 
apex of the process is in contiguity with the parietal ; a notch on each side the base of the 
process receives the anterior part of the frontal ; the angle of the outer notch touches the 
postfrontal. By its outer border the nasal unites with prefrontal, lacrymal, and pre- 
maxillary (22). The latter overlaps and conceals the naso- maxillary suture in the species 
in which such may be traced. The upper horizontal parts of the nasals gradually diminish 
to a point between the nasal portions of the premaxillaries. The exposed extension of the 
nasals terminates, in Ich,. tenuirostris, about as far in advance of the nostrils as the fore- 
boundary of these is in advance of the orbits. 
The palatines (PL XXV, fig. 1 , 20) are long, slender bones, commencing behind at 
the anterior notch between the pterygoid (24) and ectopterygoid (25), forming, as 
each advances, the mesial boundary of the small palato-naris (/j?z). The palatine then 
extends forward, joining mesially the pterygoid, until this diminishes to a point, when the 
palatines come into contact or near contact with each other at the midline of the palate. 
Externally the palatine unites with the maxillary (21), but the suture is hidden by the 
14 
