GENERAL STRUCTURE AND ORDERS. 1021 



whole animal kingdom. The similarity in the structure of the 

 vertebral column of the earlier Elasmobranch and Ganoid Fishes 

 with that of the Labyrinthodont Amphibians is also important 

 evidence pointing in the same direction.. 



This class may be divided into the four orders, Labyrinthodontia, 

 Apoda, Ecaudata, and Caudata. The first is totally extinct, the 

 second is at present unknown before the existing epoch, while we 

 have no certain record of the occurrence of the third before the 

 Cretaceous, and of the fourth previously to the Tertiary. 



Order I. Labyrinthodontia. — Since the name of this order is 

 not strictly applicable to all its members it has been proposed to 

 substitute the term Stegocephala ; but, as the same objection might 

 be taken to a large number of terms in use, such a change seems 

 unnecessary. Using, then, the Labyrinthodontia as including the 

 Ganocephala and Microsauria of some writers, its members may be 

 characterised by the following features. The body is more or less 

 elongated, and furnished with a tail ; the skull has paired supraocci- 

 pitals (fig. 950, S.O), and its postero-lateral regions are roofed over 

 by a postorbital (P.t.o) anteriorly, and a supratemporal (S.T) poste- 

 riorly. There is, moreover, very generally an epiotic 1 {E.fi) ; the 

 orbits frequently have a bony ring in the sclerotic ; and there is a 



Fig. 951. — Ventral aspect of the thoracic buckler of Actinodon Frossardi ; from the Permian 

 of France ; two-fifths natural size, ent, Interclavicle ; ep, Clavicle ; s.c/, Supraclavicle ; o, 

 Scapula. (After Gaudry.) 



parietal foramen. Palatine and vomerine teeth are very generally 

 present, and the dentine of the teeth is frequently more or less 

 folded, or plicated, from the sides. The centra of the vertebras, 

 which are amphiccelous, may be imperfectly ossified, and frequently 

 retain a notochordal canal in the middle. Usually there is a buckler 

 on the inferior surface of the thorax, consisting of one median, and 

 two lateral flattened bones, probably representing the interclavicle 

 and clavicles; the relations of these bones being shown in fig. 951. 



1 Dr Baur regards this bone as the opisthotic ; and also considers that the 

 bone here termed the squamosal is the supratemporal, and vice versa. 



