GENERAL STRUCTURE AND ORDERS. 



1029 



omerous type in the caudal. An excellent example of the rhachitom- 

 ous type of vertebra is shown in fig. 960, the pleurocentra being 

 omitted. 



It should be observed that this interpretation of the homology of 

 the rhachitomous vertebra is not accepted by Mr Hulke. 



Family Archegosaurid^e. — The well-known genus Archego- 

 saurus, ranging in Europe from the Carboniferous to the Permian, 

 may be taken as the type of a family, which for the present may 

 include most of those forms in which the dorsal vertebrae are of the 



Fig. 958.— Upper surface of the cran 

 ium of Archegosaurus ; from the Car- 

 boniferous. Reduced. Pmx, Premax- 

 illa ; Mx, Maxilla ; Na, Nasal ; La, 

 Lachrymal ; Fr, Frontal ; PF, Pre- 

 frontal ; Pa, Parietal ; PtF, Postfrontal ; 

 PtO, Postorbital ; Ju, Jugal ; QJ, Quad- 

 ratojugal; Sq, Squamosal; ST, Supra- 

 temporal ; Ep, Epiotic ; SO, Supraoccip- 

 ital. (After Miall.) 





_ Fig. 959. — Upper aspect of the 

 right carpus of Ery ops ; from the 

 Permian of North America. Re- 

 duced. _ R, Radius; U, Ulna; 

 r, Radiale ; i, Intermedium ; u, 

 Ulnare ; c 1 , c 2 , c 3 , Centralia ; 

 1-5, Carpalia; i.-v., Metacarpals. 

 (After Baur.) 



rhachitomous type throughout life. Its members have, indeed, 

 been split up into the Melosauridce, Trimerorhachidce, and Eryo- 

 pidce ; but the observations of Dr Fritsch indicate that for the 

 present at least such divisions are not definable. According to the 

 last-named authority this family may be characterised as including 

 Labyrinthodonts of medium dimensions ; having cylindrical teeth 

 of varying size, in which the folding of the dentine is comparatively 

 slight ; the upper surface of the skull being pitted ; the supraoccip- 

 itals ridged ; and the trunk vertebrae rhachitomous, and the caudal 

 usually embolomerous. A ring of bones is usually developed in the 

 sclerotic ; the ventral surface of the body is always covered with 



