528 



Vertehrata. 



diversity of form; tte crown is short and tubercalate; or furnished 

 with low transverse ridges (Mouse, Rat) ; or each has two fangs, but 

 the crown is longer and is folded both from above downward and also 

 laterally ; or again, the roots are quite short as compared with the 

 long plicate crowns ; lastly, they often grow from persistent pulps, 

 and are provided on each side with deep, perpendicular folds 



Fig". 423. Transverse sections of molars of various Rodents (at about a similar stage of 

 attrition). A Hare, B Beaver, Field Mouse, c cement, d dentine, e enamel. — After 

 Owen. 



which extend for some distance into the tooth, and are partially or 

 entirely filled with cement. On the grinding surface, therefore, there 

 are transverse or oblique stripes of enamel 

 with cement and dentine between. Occasion- 

 ally the molars with persistent pulps (c/., the 

 molars of Elephants) are even divided into a 

 series of perpendicular transverse plates with 

 cement between them. This variety in the 

 form of the teeth is correlated with a diversity 

 of habit. The molars with short crowns are 

 relatively little used, the others more or very 

 much. The number of teeth is greatest in 

 the Hares, 2^ Ij ■"* fj ™ others it is more or 

 less reduced from the anterior end of the series ; 

 from the accompanying list, to the exclusion of all the premolars;* 

 only from quite a few forms {e.g., the Australian rat, Hydromys, 

 belonging to the Muridse), one of the molars, namely, the last m?, is 

 also absent. 



Whilst the articular facets for the lower jaw in most Mammalia 

 are in the form of transverse surfaces or pits, in most Rodents there 



even, as may be seen 



* As the correspondin,'^ milk: teeth are also generally absent, and as (with the 

 exception of the Hares) the incisors have no predecessors, theje is sibsQlutely ng 

 replacement in forms destitute of premolars, 



