THE SKELETON OF REPTILES 25 



an axiom that new bones have not appeared in the skulls of reptiles, 

 birds, or mammals; and that no bone which has once disappeared 

 has ever been functionally regained by the descendants of those 

 that lost it. The presence, then, of an extra bone in the temporal 

 region of the lizards or the ichthyosaurs is proof that they have had 

 a long and independent descent from reptiles which possessed it. 



The mandible of the earliest reptiles was composed of not less 

 than seven separate and distinct bones, as shown in the accom- 

 panying figures. The mandible of no modern reptile has more 

 than six, and some have fewer. The mandible of mammals is 

 composed of a single bone, the dentary; those reptiles, the Therio- 

 dontia, which doubtless were ancestral to the mammals in Triassic 

 times, have all the bones, except the dentary, much reduced, or 

 even vestigial. The prearticular bone, as shown, so far as known, 

 has been absent in all reptiles since Triassic times, except the 

 ichthyosaurs, plesiosaurs, Sphenodon, and turtles, all reptiles of 

 ancient origin. The coronoid bone primitively extended the whole 

 length of the teeth on the inner side; in all reptiles, except the 

 plesiosaurs, since Triassic times it is either reduced to a small bone 

 back of the teeth or is absent. So also the splenial has been greatly 

 reduced in size in all later reptiles and may be wanting as in 

 Sphenodon and modern turtles. The articular of reptiles, it is now 

 generally believed, is represented in mammals by one of the ear 

 bones, the quadrate by another. 



The teeth of reptiles are of much less importance, as a rule, in 

 the determination of relationships than are the teeth of mammals. 

 Rarely are their shapes of specific, and often not of generic, impor- 

 tance, though their number and relative sizes may be. The teeth 

 of mammals, as a rule, are forty-four or less in number, and they 

 are always inserted in distinct sockets in the jaw bones. Among 

 reptiles they are indefinite in number, and may be attached to 

 any of the bones of the palate and sometimes also to the coronoid 

 of the mandibles. Furthermore, except in those reptiles related 

 to the immediate ancestors of the mammals, they are alike or 

 nearly alike in the jaws, that is, homodont, not distinguishable 

 into incisors, canines, and molars. They may be inserted in 

 separate sockets (thecodont), in grooves, or simply be co-ossified 



