Glass 3. Amphibia. Order 1 . Urodela. 



405 



metamorphosis {Amhly stoma mexicanum) is similar to a Salamander. The 

 Axolotl, wliicli is indigenous to Mexico, is oviparous ; when first hatched, it is 

 just like the triton larva of the same stage. 



4. Under the name, Perennibranchiata, are collected all the Urodela 

 described above (p. 403) as retaining gills and other larval characteristics 

 thi'oughont life. Amongst these is the blind, pale, elongate Proteus anguineus, 

 which has rudimentary eyes, and three digits on each foot ; in subterranean lakes in 

 Austria. Furthermore, the genus, Menohranchus, less elongate, with four digits 

 on each foot, and Siren lacertina, which may attain a length of 1 m., 'with 

 homy jaws ; vermiform, without hind limbs ; both in W. America. The genera, 

 Menopoma and AmpMuma (the latter vermiform ; with four very small limbs, each 

 with two or thi-ee digits), lose their gills as ah-eady mentioned, but retain the gill- 

 slits and several other laiTal characters. Nearly allied to Menopoma, is the 

 Japanese Giant Salamander {Cryptobranchus japonicus), in which the 

 branchial clefts are closed. 



Related to the living Urodela are the StegOCephala (primitive Amphibia), a 

 large gi'oup, which lived in the Carboniferous, Permian, and Triassic periods, and 

 of which a few were remarkable for their great size ; skulls are known of I'S m. 

 long. The skull has a larger nuniber of membrane bones than in existing 

 Amphibia ; there is, for example, a double supraoccipital and several others.* 

 The skull bones are often scarred externally, and this signifies that they were 

 located close below the sm-face, covered only by a thin skin ; sometimes there 

 are furrows on the head, for the branches of the lateral line,t recalling those 



A B ■ C 



o 





'^ 



If 



^<F=?SHV 



WjcV 



Fig. 330. Skull of a, Stegooephalon (Tre- 

 matosaurus), from below (A), from above (B), and 

 from the side (0). 1 Oi-bit, 2 external nares, 3 internal' 

 nares, 4 foramen magnum, a occipital, b parietal, 

 c frontal, d paraspbenoid, g palatine and pterygoid, 

 i nasal, v vomer, a occipital condyles. The other 

 letters disting-uish various membrane bones. 



of many Pish. As in the Amphibia of to-day there were two occipital condyles. 

 The notochord was, to a large extent, persistent, the centra often biconcave ; 

 ribs sometimes long.J Some Stegooephala have five digits on the fore Umbs. 

 The sclerotic coat of the eye (unlike that of existing Amphibia) usually had a 

 ring of bony plates. In the skin of the ventral surface (rarely on the dorsal) 



* Between the parietals there is often a fairly large parietal foramen, which 

 indicates the presence of a parietal eye (see p. 337). 



t The head often forcibly recalls that of the bony Ganoids. 



episternum and a 



J The sternum was cartilaginous, but there was an 

 clavicle, like those of the Lacertilia. 



