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Tribe II. IMMUTABILIA. 



CHARACTERS. 



I. The animals of this tribe undergo no metamorphosis, but come into hfe with 

 the forms they are permanently to retain. 



II. The body is elongated and cylindrical. 



III. The tail is long, round, or compressed, and sometimes ancipital. 



IV. The extremities vary in number; in some there are four, (Menopoma,) 

 in others only two, (Siren.) 



Remarks. This tribe of reptiles embraces some of our most remarkable 

 species, as the Siren, Lacertina, Menobranchus, Lateralis, &c. Fitzinger has 

 ao-ain subdivided this tribe into two families, each of which includes two or more 

 genera. 



Family I. CRYPTOBRANCHOIDEA. 



CHARACTERS. 



The reptiles of this family are destitute of gills at all periods of their existence, 

 and breathe by means of exposed spiracles or branchial orifices at the neck. 



