MOUTH AND NOSE 



Among less modified sorts the rodent Ichthyomys has the lateral 

 edge of the upper incisors prolonged into a sharp point. This un- 

 doubtedly facilitates the capture of small fish, but the development may 

 have been fortuitous^ for the members of the terrestrial subgenus 

 Mictomys exhibit the same character. It is caused by the very slender 

 lower incisors working against the broader upper ones, leaving the 

 lateral edges largely unworn. The upper incisors of the beaver are 

 especially modified to act as chisels for cutting down trees a character 

 that really has no bearing on aquatics. 



Two aquatic mammals other than whales lack teeth in the adult 

 state. Both the platypus and Steller sea cow ( ' Hydromadalis ) shed 

 the molars at a very tender age, their places being taken by leathery 

 or horny plates a development accompanying the food habits of the 

 former, feeding upon worms and similar soft foods, and of the latter 

 in consuming soft marine algae. In the young Halicore there are four 

 pairs of teeth beneath the horny plate of the mandible but these are 

 absorbed before maturity. In this genus a pair of the upper incisors 

 have developed into small tusks in the male, but these are diminutive 

 and non-protruding in the female. In the manati (Trichechus) in- 

 cisors are said to be present only during the fetal stage. In this genus 

 but in no other sirenian, either living or extinct, there is a progressive 

 succession of the molariform teeth comparable to conditions in the ele- 

 phant. This takes the form of a constant forward displacement and 

 loss of the tooth from the front of the series as a new back one de- 

 velops. After studying a considerable series of skulls of different ages 

 Thomas and Lydekker (1897) came to the conclusion that during the 

 span of a manati's lifetime there must be not less than 20 teeth in 

 each series, which is the same as stating that this process of succession 

 will continue ceaselessly until death, or at least until senescence. If 

 this be the case the teeth can hardly be homologized with those of the 

 normal mammalian complement, the whole process of succession being 

 different. 



Another interesting alteration of the molariform teeth is that en- 

 countered in the walrus, which feeds largely upon mollusks, and the 

 back teeth have become very broad and flat for crushing these. Some- 

 what similar in general respects are the teeth of the sea otter, which 

 favors echinoderms or sea urchins as an article of diet. The molars 

 of seals and sea-lions are simpler than those of the average terrestrial 

 carnivore, although whether they have always been so or have become 

 secondarily simplified from a more complex pattern has not been estab- 



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