576 CLASS xvn. 



We have already said that the mammals have two jaws; but 

 the under-jaw alone is moveable. In man this motion is threefold; 

 first the jaw can be depressed and again raised, next can be 

 extended forward and again retracted, and lastly can be moved 

 obliquely to the left side and the right, whilst one of the condyles 

 rotates in the articular cavity of the temporal bone as round its 

 axis, and the other slides somewhat out of its cavity forward and 

 inward. These three motions are rendered possible by the round- 

 ness of the condyle and the slight depth of the articular cavity. 

 In the mammals the motions are usually more limited. In the 

 carnivores the articular condyle is broad, and is received in a deep, 

 transverse groove placed between projecting lines, in such a way 

 that motion forward and backward is prevented, whilst the under- 

 jaw in dilacerating animal food is principally capable of powerful 

 elevation and depression. In the rodents the condyle is longitudi- 

 nal, in the same direction as the long dimension of the lower-jaw, 

 and is received in a spacious, shallow cavity below the malar pro- 

 cess of the temporal bone; hence the under-jaw can glide forward 

 and backward with facility, which in gnawing their food constitutes 

 its principal motion. In the ruminants, finally, the articular 

 groove is very shallow, the condyle transverse and flat, and the 



are the molars properly so called." OWEN in TODD'S Cyclop, iv. p. 903. The typical 

 dentition of diphyodonts as determined by OWEN from the forms of mammalia first 

 introduced into this planet, whether carnivorous or graminivorous, was 3 incisors, 

 i canine, and 7 succeeding teeth above and below. Three of the seven may be pre- 

 molars and four may be true molars, or four may be premolars and three true molars ; 

 the first being a character of the placental diphyodonts, the second of the marsupial. 

 True molars are a continuation of the first set of teeth backward, and these are all 

 developed in the same primary groove of the foetal gum. The successional teeth, or 

 the premolars, are formed from tooth-germs developed on the side of the deciduous 

 teeth, and therefore grow in a secondary groove. These secondary tooth-germs are 

 three in number, in one group only. When the premolars and molars are below the 

 typical number the absent teeth are missing from the fore part of the premolar series, 

 and from the back part of the molar series, the most constant teeth being the fourth 

 premolar and the first true molar. These having been determined in any case by 

 observation of the development, the rest of the premolars are counted from the last 

 forward (4, 3, 2, i), of the molars from the first backward (i, 2, 3). The typical den- 

 tition is expressed by the following formula, j.-lHl, c. I I , jP. -, m.^~^ = 42, 



3~3 J J 4~4 33 



for placental diphyodonts. OWEN 1. 1. and also On the Characters of the class Mammalia, 

 in Proceedings of the Linn. Soc. n. No. 5, 1857, pp. 7 9.] 



1 [The order Monotremata, Edentata and Cetacea generate a single set only of teeth 

 (Monophyodonts OWEN); all the rest generate two sets (Diphyodonts OWEN).] 



