38 THE VOYAGE OF H.M.S. CHALLENGER. 



and the palatine ; there is no lachrymal bone. The existence of a distinct nasal bone is 

 mentioned by Owen (Eeport of Brit. Assoc, 1846, p. 224) in the existing Hydromedusa, 

 and in the fossil forms — Chelone lolanicei^s and Chelone indchriceps). 



The j)remaxillaries (figs. 1-3, px.) are remarkable for their direction, which is rather 

 inferior than anterior ; they have a sharp dentary margin and short palatine processes. 

 The maxillaries (onx) are very large and Mammalian, so to speak, with a high facial, a 

 considerable palatine, and an extended jugal region ; their dentary edge is sharp or 

 cultrate, and is denticulated in relation to the large papillae on which the bony sheath is 

 developed. The single vomer {v.) is like what is seen higher up, viz., in the Falcons, having 

 a lower palatine plate, helping to form the hard j)alate, an ascending ploughshare part, and 

 a thin scooped upper edge for adaptation to the " orbito-nasal septum." The broad flat 

 palate is largely formed by the palatines and pterygoids — membrane bones — but so inti- 

 mately connected with the endo-skeletal structures as to be worthy to be classed with them. 

 Here, again, the Mammalian skull is being prefigured, for the palatines (fig. 3, pa.) have 

 a considerable region on the lower plane that makes the hard palate ; this is carried to 

 excess in the Crocodilia, where the pterygoids — as in Myrmecojyhaga — also contribute 

 to this lower secondary floor. The upper part of these bones is like the thin shell of a 

 bivalve ; the right and left bones are kept apart by the upper limb of the vomer ; behind, 

 their sinuous edge articulates with the fore edge of the pterygoids. 



The pterygoids (jyg-) are essentially lunate bones, placed back to back ; their broad 

 part is in front, where they meet at the mid-line ; behind, they diverge considerably, and 

 each bone, reduced to two-thirds of its front width, clamps the base of the skull, and is 

 applied as a splint to the inner face of the quadrate (q.). The outer edge is concave and 

 bevelled, the lower face a little concave, and the upper slightly convex ; in the re-entering 

 angle between the two bones, below, the basisphenoid (b.s.) is exposed ; there is no 

 "parasphenoid" here. 



The orbital rim is well formed already, the frontal, prefronto-nasal, and maxillary form 

 the front half, the hind part is nearly all completed by the jugal (j.) below, and the post- 

 orbital (pt.o.) above ; the former is a falcate bone, with a facial and an orbital lamina, 

 and so is the latter, but it is much broader. According to the ancient imbrication of 

 these scales, the maxillary overlaps the jugal, and the jugal the post-orbital ; this, in its 

 turn, overlaps the squamosal (sq.) behind it, and the jugal overlaps a second plate, the 

 " quadrato-jugal" (q.j-), a thin scale of bone, whose concave hinder edge forms the fore 

 margin of the tympanic ring, by lying as a splint exactly on the outer face of the 

 quadrate. 



The squamosal does the same for the postero-inferior edge of that space ; behind, it 

 is thick and two-edged (figs. 2, 5, sq.) ; there is no additional "supra-temporal" bone 

 here, such as we see in the " Lacertilia." 



The free mandible has only five bony plates upon it ; the " splenial" is absent ; the 



