LIASSIC ICHTHYOSAURS. 57 



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 Icldhyosaurus, 

 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 IcIdJii/osanrus. 



The nasals (Tabs. XIX, XXIII, figs. 1, l5) 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 oi'bits. 



The palatines (Tab. XXI, 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 {pn). 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 

 underlnpping extension of the palatal process of the premaxillary (22), which reaches to 

 the palato-naris. 



The maxillary (Tab. XX, fig. 1, 21) begins behind, parallel with the anterior third 

 of the orbit, from which it is divided by the slender fore part of the malar (26). As it 

 advances the maxillary expands into a facial, an alveolar, and a palatal plate, articulating 

 mesially with the ectopterygoid (Tab. XXI, fig. 1, 25), and, in advance of the palato-naris, 

 with the premaxillary (22). The palato-alveolar part of the maxillary is divided from 

 the facial part by the outer alveolar wall. The facial part (Tab. XX, fig. 1, 21), coming 

 into view beneath the fore part of the malar (26), expands slightly to unite with the 

 lacrymal (13), and, in the species affording the subject of fig. 1, Tab. XX, it contributes 

 the middle third of the lower border of the nostril ; but, as a rule, it is separated there- 

 from by the junction of the lacrymal (73) with the premaxillary (22), as in fig. 2. In 

 advance of this the maxillary is overlapped by the premaxillary, which conceals it from 

 view at about the fifteenth tooth, counting forward, in Icldhyosaurus tenuirostris. In no 

 species does it support more than about one third of the series of teeth on its own side 

 of the upper jaw. 



The premaxillary (Tabs. XIX— XXI, XXV, figs. 1, 22) is characterised by its great 

 8 



