24 



ated.* The blood is cold, the circulation incomplete and 

 practically suspended during hibernation. There are no 

 external scales or scutes (Amphiuma and Caecilia except- 

 ed), the skin being- smooth and naked. This characteristic 

 will enable the veriest novice to distinguish these animals 

 from all lizards. 



The foetus is without the embryonic sac known as the 

 amnion, and the allantois (organ by which foetal blood is 

 aerated) is absent, but represented by the urinary bladder. 

 The skeleton is internal, the vertebrae biconcave (amphi- 

 ccelous), or concave behind and convex in front (opistho- 

 ccelous). No Salamander has vertebrae which are concave 

 in front only (proccelous), as in frogs. In the development 

 the vertebrae are at first amphiccelous, as in fishes, an ossi- 

 fication of the intervertebral cartilage attaching itself later 

 on to form those vertebrae which are opisthocaelous. The 

 skull is connected by two occipital condyles, and the nasal 

 sacs open posteriorly into the pharynx. The reproductive, 

 urinary, and digestive organs open into a common recepta- 

 cle, the cloaca. 



The Sirens have fore feet only. All other tailed am- 

 phibians have four limbs, in which the radius and ulna, and 

 tibia and fibula, are not anchylosed as in frogs. The 

 Sirens of our Southern States and the Mud Puppies, or 

 Water-dogs, of Western Rivers, with the Proteus of Aus- 

 tria, are the only forms which are perennibranchiate, the 

 Siredons, or Axolotls being now accepted as more or less 

 persistent larval forms of Amblystoma. 



The Congo Snakes of the South and Hellbenders, or 

 Mud-devils, of Ohio, retain branchial apertures in the neck 

 of the adult. All other Salamanders of the United States 

 are caducous, the gill slits being perfectly closed in adult. 



The famiiy divisions are based principally upon the ar- 

 rangement of the teeth and the generic upon the shape of 



* Professor John Michels has recently demonstrated the existence of both a 

 nucleus and a nucleolus in mammalian red blood corpuscles. — Set. Am. Supp.. 

 May 4, 1895, p. 16,126. 



