384 



RODENTIA. 



and other Rodents with rootless molars, the 

 reproduction of the molar, like that of the in- 

 cisor teeth, appears to continue throughout 

 the animal's existence. The rootless and per- 

 petually growing molars are always more or 



already been cited ; but in the rootless molars, 

 where the folds of enamel extend inwards from 

 the entire length of the sides of the tooth, the 

 characteristic configuration of the grinding 

 surface is maintained without variation, as in 



Fig. 2G8. 



Upper jaw of the Patagonian Cavy (Chloromys Patagonica). 



i, incisor tooth, laid bare throughout its whole length ; m, p, molar teeth implanted without fangs into 



arched sockets. 



less curved ; they derive from this form the 

 same advantage as the incisors, in the relief 

 of the delicate tissues of the active vascular 

 matrix from the effects of the pressure which 

 would otherwise have been transmitted more 

 directly from the grinding surface. 



The complexity of the structure of the 

 crown of the molar teeth, and the quantity of 

 enamel and cement interblended with the 

 dentine, are greatest in the rootless molars of 

 the strictly herbivorous Rodents. The crowns 

 of the rooted molars of the omnivorous rats 

 and mice are almost as simple as the tuber- 

 eulate molars of the bear, or of the human 

 subject, which they appear to typify. They 

 are at first tuberculate ; when the summits of 

 the tubercles are worn off, the inequality of 

 the grinding surface is for a time maintained 

 by the deeper transverse folds of enamel, the 

 margins of which are separated by alternate 

 valleys of dentine and cement ; but these 

 folds, sinking only to a slight depth, are in 

 time obliterated, and the grinding surface is 

 reduced to a smooth field of dentine, with 

 a simple border of enamel. A similar 

 change in the grinding surface, consequent 

 on age and use, is shown in the molars 

 of the souslik, or ground squirrel ; as also 

 in those of the gerbille, and is common 

 to all that possess roots. It will be seen that 

 these folds have a general tendency to a trans- 

 verse direction across the crown of the tooth. 

 Baron Cuvier has pointed out the concomitant 

 modification of the shape of the joint of the 

 lower jaw, which almost restricts it to horizontal 

 movements to and fro, in the direction of the 

 axis of the head, during the act of mastication. 

 When the folds of enamel dip in vertically 

 from the summit to a greater or less depth 

 into the substance of the crown of the tooth, 

 as in those molars which have roots, the con- 

 figuration of the grinding surface varies with 

 the degree of abrasion, of which examples have 



the Guinea-pig, the capybara, and the Pata- 

 gonian cavy. 



The whole exterior of the molar teeth of 

 the Rodentia is covered by a cement, and the 

 external interspaces of the enamel folds are 

 filled with the same substance. In the chin- 

 chilidae ami the capybara, where the folds of 

 enamel extend quite across the body of the 

 tooth, and insulate as many plates of dentine, 

 these detached portions are held together by 

 the cement ; such folds of enamel are usually 

 parallel, as in the large posterior lower molar 

 of the capybara, which, in shape and structure, 

 offers a very close and interesting resemblance 

 to the molars of the Asiatic elephunt. 



The partial folds and islands of enamel in 

 the molars of the porcupine and agouti, 

 typify the structure of the teeth of the rhi- 

 noceros ; the opposite lateral inflections of 

 enamel in the molars of the gerbille and Cape 

 mole-rat represent the structure of the molars 

 of the hippopotamus; the double crescentic 

 folds in the jerboa sketch out, as it were, the 

 characteristic structure of the molars of the 

 Anoplothere and Ruminantia. 



Although, as has been shown, the molar 

 teeth in many Rodents are rootless and of un- 

 limited growth, as in the Edentata, in none 

 is enamel absent ; or vascular dentine, as the 

 chief constituent of the tooth, present. These 

 essential differences characterise the molars 

 of those Rodents, which by use have their 

 grinding surface reduced to a simple depres- 

 sion bounded by a raised circular margin, as 

 in the great Cape mole; that margin being 

 formed by true enamel, but in the sloths by 

 hard dentine. 



It is peculiar to some of the Rodents with 

 rootless molars to have the sockets of these 

 long curved teeth open at both extremities, so 

 that, in the dry skull, the base of the tooth 

 protrudes as well as the grinding surface ; the 

 matrix in such instances adheres to the peri- 



