A, C, D, E, THEROMORPHA. A, _Elginia_, p. 305; C, _Cynognathus_, p. 306; D, _Gordonia_, p. 310; E, _Dicynodon_, p. 310. B, G, PROSAURIA. B, _Sphenodon_, p. 294; G, _Palaeohatteria_, p. 291. F, CROCODILIA, p. 434. H, I, K, CHELONIA, p. 316. H, _Chelydra_, p. 338; I, _Chrysemys_, p. 346; K, _Cistudo_, p. 361. _E_, Epiotic; _F_, frontal; _IT_, infratemporal fossa; _J_, jugal, shaded vertically; _L_, lacrymal; _M_, maxillary; _N_, nasal groove; _Na_, nasal bone; _O_, orbit; _P_, parietal; _Po_, postorbital, dotted; _Pf_, post-frontal; _Pm_, premaxillary; _Pr_, prefrontal; _Ptg_, pterygoid; _Q_, quadrate; _Qj_, quadrato-jugal; _So_, supra-occipital; _Sq_, squamosal, shaded obliquely; _St_ (in B-E), supratemporal fossa; _St_ (in A), Supratemporal bone.] {281}[Illustration: FIG. 55.–Diagrams of skulls, showing especially the composition of the bony arches of the orbito-temporal region. L, PYTHONOMORPHA. _Clidastes_, p. 490. M, N, O, LACERTILIA, p. 496. M, _Varanus_, p. 543; N, _Uromastix_, p. 524; O, _Lacerta_, p. 550. P, ICHTHYOSAURIA, p. 479. _Ichthyosaurus_, p. 483. Q, PTEROSAURIA, p. 484. _Dimorphodon_, p. 486. R, AVES, generalised, for comparison. S, MAMMALIA, generalised, for comparison. T, OPHIDIA, p. 581. _C_, Condyle of mandible; _Col_, columella cranii; _F_, frontal; _I_, interparietal or pineal foramen; _I.A_, Inner angle of mandible; _J_, jugal, shaded vertically; _L_, lacrymal; _M_, maxillary; _N_, nasal groove; _Na_, nasal bone; _O_, orbit; _O_{1}_, preorbital fossa; _P_, parietal; _Pf_, postfrontal; _Pm_, premaxillary; _Pr_, prefrontal; _Ptg_, pterygoid; _Q_, quadrate; _Qj_, quadrato-jugal; _Sq_, squamosal, shaded obliquely; _St_, supratemporal bone.] {282}The families cannot well be changed, and terms like super-families and super-orders are sometimes resorted to by those who do not like to look stern facts in the face. The sequence of the groups, although arranged as much as possible in ascending order, is of necessity as unnatural as that of the maps in an atlas. We cannot yet construct a satisfactory phyletic tree of the Reptiles. The Proreptilia connect them with the Amphibia. Next follow the Prosauria with _Sphenodon_ among the Prosauri as the key to most other groups. Then follow the Theromorpha, and it is probable that from various branches of these have arisen the Chelonia, Dinosauria, Crocodilia, and Plesiosauria. The descent of the Ichthyosauria is very problematic. The same applies to the Pterosauria and to the Pythonomorpha, but it is possible that they, together with the Sauria, are connected with the Prosauria. With all reserve these hypothetical affinities may be expressed by the following diagram:–