28 GENERAL ANATOMICAL CHARACTERS 



Pig, which we generally write as i %, c \, p f, m|. Such a 

 generalised formula will admit of modification into that of all 

 existing, and a large number of fossil Marsupials, but it is possible 

 that some of the Mesozoic types may have had more than four 

 premolars, although there is no absolutely decisive evidence that 

 such was the case. The presence of seven or eight true molars in 

 some Mesozoic forms merely entails the addition of two or three 

 additional figures to the ideal generalised formula. 



The milk-dentition of all known Marsupials, existing or extinct, 

 is (if not entirely absent) limited to a single tooth on either side of 

 each jaw, this being the predecessor of the last permanent premolar. 

 And if the view that the milk -dentition is an additional series 

 grafted upon the original permanent series be correct, it is evident 

 that we have in this single replacement the first stage of this 

 additional development. 



In very few mammals are teeth entirely absent. Even in the 

 Whalebone Whales their germs are formed in the same manner 

 and at the same period of life as in other mammals, and even 

 become partially calcified, but they never rise above the gums, 

 and completely disappear before the birth of the animal. In some 

 species of the order Edentata, the true Anteaters and the Pangolins, 

 no traces of teeth have been found at any age. The adult 

 Monotremata are likewise devoid of teeth of the same structure 

 as those of ordinary mammals ; but well-developed molars occur in 

 the young Ornithorhynchus, although no traces of teeth have hitherto 

 been detected in Echidna. 



Modifications of the Teeth in Relation to their Functions. — The 

 principal functional modifications noticed in the dentition of 

 mammalia may be roughly grouped as piscivorous, carnivorous, 

 insectivorous, omnivorous, and herbivorous, each having, of course, 

 numerous variations and transitional conditions. 



The essential characters of a piscivorous dentition are best 

 exemplified in the Dolphins, and also (as modifications of the 

 carnivorous type) in the Seals. This type consists of an elongated, 

 rather narrow mouth, wide gape, with numerous subequal, conical, 

 sharp-pointed, recurved teeth, adapted simply to rapidly seize, but 

 not to divide or masticate, active, slippery, but not powerful prey. 

 All animals which feed on fish as a rule swallow and digest them 

 entire, a process which the structure of prey of this nature, especially 

 the intimate interblending of delicate, sharp-pointed bones with the 

 muscles, renders very advantageous, and for which the above- 

 described type of dentition is best adapted. 



The carnivorous type of dentition is shown in its most specialised 

 development among existing mammals in the Felidce. The function 

 being here to seize and kill struggling animals, often of large size 

 and great muscular power, the canines are immensely developed, 



