472 BRITISH FOSSIL REPTILES. 



require considerable force of jaw for overcoming its struggles. The means of resist- 

 ance were afforded to the upper or fixed maxilla, not by a continuous wall of bone, but 

 by curved columns or abutments. The chief of these is the upper medial arch of bone 

 which overspans the skull lengthwise, from the short roof of the cranium to the 

 fore part of the premaxillary (22); the froiitals (11) and nasals (15) combining with the 

 mid-fork or branch of the premaxillary (22',) to constitute this arched key-ridge of the roof 

 of the head. 



From it two piers or buttresses out-span on each side, to give strength and resistance 

 to the upper jaw, and especially its alveolar tracts. One, proceeding from the 

 nasal, meets the uprising process of the maxillary (21) ; this abutment, Curving from 

 above outward and obliquely forward, expands and backs the part of the jaw where the 

 second group of large laniaries project. The second buttress is continued from the pre- 

 frontal (14), and arches more directly outward to meet the uprising process of the 

 malo-maxillary. A third arch, due to the post-frontal (12) and malar (26), expands to abut 

 upon the hind end of the maxillary arch, and gives support to the part of the skull which 

 the temporal muscles tended to pull downward when they were giving to the mandible 

 the power of a strong bite or grip. Finally, comes the strongest of the four piers, due to 

 the mastoid (s) and tympanic (2S), for giving articular attachments to the rami of the 

 lower jaw. 



Thus, four vacuities appear in the side-walls of the skull : the first („) is the largest, 

 between the small consolidated or continuous fore part of the skull (22), and the naso- 

 maxillary pillar (21% 15). This vacuity answers to the external bony nostril of the same side, 

 in the Lizard's skull (PI. 17, fig. 3, «), where the nostrils are divided and more or less lateral. 

 The second vacuity («) is somewhat less, ofa triangular form, with the base downward : it 

 answers to the antorbital vacuity in Lyrioceplialus (ib., a) and a few other Lizards, and to that 

 in Teleosaurus, where, however, it is very small. The third vacuity (o) still decreasing, is 

 oval, with the narrow end or apex downward : it answers to the orbit, but is of large size 

 compared with most Saurians ; it is, however, exceeded in relative expanse by the orbit in 

 Lyrioceplialus (ib., 0). 



The fourth vacuity is the narrowest : it answers to the so-called ' temporal fossa ' and 

 was occupied by the muscles of the same name. Extension of surface, for their origin, and 

 additional strengthening of this back part of the skull are gained by laying horizontally 

 across the temporal fossa the bony beams called ' upper and lower zygomata,' arching 

 from the postfronto-malar to the masto-tympanic vertical columns. The heavy phosphate 

 of lime, thus singularly economised by the disposition of the bones on mechanical prin- 

 ciples plainly to that end, is made to go still further by the arrangement of the osseous 

 tissue. Every bone is pneumatic, the abundant, open, cancellous structure being included 

 in a very thin layer of compact osteine. 



The bones of the limbs are dislocated and dispersed in the way and degree common 

 to the specimens of this animal hitherto discovered (Buckland, loc. cit., pi. xxvii; and pis. 



