LIASSIC FORMATIONS. 99 



Crocodiles and Lizards ; in these, it is limited to a small part of the dentary element, 

 but in Ichthyosaurus 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. Thus, the latter is longer than the dentary symphysis mlch. 

 intermedius, but is shorter in Ich. communis, and, relatively, much shorter in Ich. tenui- 

 rostris (PI. XXV, fig. 3), Ich. lonyirostris (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 Pishes, 

 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 (PI. 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 (p n). 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 

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