ORDERS OF AMPHIBIA. 443 



are biconcave (amphiccelous). " In the thoracic region three 

 superficially-sculptured exoskeletal plates, one median and two 

 lateral, occupy the place of the interclavicle and clavicles. 

 Between these and the pelvis is a peculiar armour, formed of 

 rows of oval dermal plates, which lie on each side of the 

 middle line of the abdomen, and are directed obliquely for- 

 wards and inwards to meet in that line" (Huxley). 



The head was defended by an external covering or helmet 

 of hard and polished osseous plates, sculptured on their exter- 

 nal surface, and often exhibiting peculiar, smooth, symmetrical 

 grooves the so-called " mucous canals." The skull was arti- 

 culated to the vertebral column by two occipital condyles. 

 The teeth are rendered complex by numerous foldings of their 

 parietes, giving rise to the "labyrinthine" pattern, from which 

 the name of the order is derived. 



The Labyrinthodonts are known to occur from the Carboni- 

 ferous to the Triassic or Liassic period inclusively ; but they 

 are most characteristically and distinctively Triassic. 



DISTRIBUTION OF AMPHIBIA IN TIME. From a geological 

 point of view, by far the most important of the Amphibia are 

 the Labyrinthodontia, the distribution of which has just been 

 spoken of. The living orders of Amphibia are of much more 

 modern date, being, as far as known, wholly Tertiary and 

 Post-tertiary. The Anoura are represented by both Toads 

 and Frogs in Miocene times, and they have survived to the 

 present day. The " Tailed " Amphibians are best known to 

 geologists by a singular fossil, which was described by its ori- 

 ginal- discoverer as human, under the name of Homo diluvii 

 testis. The fossil in question is of Miocene age, and it is now 

 known to belong to a Salamander, nearly allied to the giant- 

 salamander of Java (Mcnopoma). It is termed the Andrias 

 Scheuchzeri. If the Palceosiren of Geinitz be not truly re- 

 ferable to the Labyrinthodonts, then the Urodela date from the 

 Permian period. 



