ANATOMY OE VETITEBEATES. 101 



tusk, s, wlien the month is closed. The symphysis of the man- 

 dible is peculiarly massive — broad, high, and thick. Anteriorly 

 it is convex in every direction ; it is bent or produced upward, 

 terminating in a broad trenchant margin, like the fore part of the 

 lower mandible of a macaw. The modification of the back part 

 of the cranium, especially the great expansion due exclusively to 

 the developement of ridges for augmenting the surface of attach- 

 ment of muscles (for the brain of the cold-blooded reptile would 

 need but a small spot of the centre of the occipital plates for its 

 protection), indicates the power that was brought to bear upon 

 the head as the framework in which were strongly fixed the two 

 large tusks. The strength or resistance of the cavities receiving 

 the deeply implanted bases of the tusks was increased by the 

 ridges developed from the outer part of their l^ony wall. 



Only the Crocodiles now show a like extent of ossification of the 

 occiput, and only the Chelonians the trenchant toothless mandible ; 

 but in both the outer nostril is single and median : the Lizards 

 rep)eat the divided apertures for resjiiring air : in Mammals alone 

 do we find a developement of canine tusks like that in the 

 Dicynodonts. 



§ 37. Skull of Pterosauria. — The skull of the Pterodactyle, 

 fig. Ill, was as remarkable for its lisi'ht and delicate structure as 

 that of the Dicynodont for its comjjact massiveness. It had a 

 single occipital condyle : a post-fronto-mastoid arch and a malo- 

 squamosal arch on each side ; the latter abutting against the end 

 of the tympanic pedicle. The orbit was large, and the eyeball 

 defended by sclerotic plates. The external nostrils were di^dded, 

 and placed about midway between the orbits and the muzzle. 

 There was a large vacuity between the orbits, o, and nostrils, n. 

 The jaws varied much in length in different species. 



§ 38. Scapular arch and ajypendage. — Parts which project 

 from the body to act on the surrounding medium commence as 

 a bud or fold of skin, within which is formed the framework, in 

 texture and structure according to the work to be done. The 

 reaction of the medium, whether air, water, or earth, calls for the 

 due resistance usually afforded by junction of the projecting part 

 with a segment of the endoskeleton. Thus, in Fishes, the frame 

 of the opercular flap articulates with the tympano-mandibular 

 arch : that of the branchiostegal (gill-covering) flap with the 

 ha3mal arch of the parietal vertebra : that of the pectoral flap or 

 limb with the same arch of the occipital vertebra. The frame 

 of the caudal flap or fin is attached to the terminal vertebra; of 

 the body : those of the dorsal and anal fins are less firmly inter- 



A'OL. I. M 



