vii.] THE EXTERNAL SKELETON. 271 



32. The SUCCESSION OF THE TEETH which we meet with 

 in man is characteristic of most animals of his class, but 

 not all. In many, as in the Guinea-pig, the so-called milk- 



FIG. 248. VERTICAL, LONGITUDINAL SECTION OF THE POISON-FANG OF A 

 SERPENT. {After Owen.. } 



g t deep groove ; o, its lower termination, which affords exit to the poison ; 

 p, pulp-cavity. 



teeth are shed even before birth. On the other hand, they 

 may be retained for a relatively longer period than they are 



FIG 249. MAGNIFIED TRANSVERSE SECTION OF A SERPENT'S POISON-FANG. 

 (After Owen. ) 



Z, groove round which the substance of the tooth (containing /, the pulp-cavity) 

 is bent ; j, the point where the sides of the tooth meet and convert the 

 "groove" into what is practically a central cavity. 



in man. Thus, in the Ungulates they persist till the adult 

 form is reached, and, at least in some, till after the bony 

 elements of the limb-girdles have completely coalesced. 



Sometimes teeth are formed which are not destined to 

 cut the gum, and are re-absorbed without ever becoming 

 visible. This is the case with the upper incisor teeth of 

 Ruminants. In the Whalebone Whales, before the develop- 

 ment of baleen, minute teeth are developed in the dental 

 groove, which teeth are ultimately absorbed while the- groove 

 itself becomes obliterated. 



In the Marsupial Mammals but a single tooth is provided 

 with a vertical predecessor. This tooth is the one which 

 would be the fourth pre-molar were the normal number de- 

 veloped, and it thus serves to define the last of the pre-molars. 



