402 THE ZOOLOGIST. 



observation, has always saved them from the ignominy reserved 

 to cetaceans of being generally classed with fishes. 



Another striking point of affinity in which they differ from 

 other marine mammals is that they all bring forth and suckle 

 their young out of the water, for which purposes they resort 

 to rocks, beaches, or ice ; and it is on this necessity in their 

 economy that the hunters place their principal reliance for a 

 supply. These rendezvous, to which many of the species resort 

 in vast numbers, are sedulously sought for, and in some cases 

 carefully preserved. In others indiscriminate slaughter ' has 

 sometimes destroyed the whole species inhabiting the locality. 



The characters of their teeth differ entirely from those of 

 the Sirenia which have broad or fiat-topped teeth, suitable for 

 vegetable feeding, and closely approximate the terrestrial Car- 

 nivora, on the one hand, whilst on the other they show a slight 

 tendency to degenerate towards the dentition of the Dolphins, 

 and as this point is interesting as suggesting a possible deriva- 

 tion for the latter cetaceans, it will be dealt with a little more 

 in detail. 



In the skull of an Icthyosaurus we have a long series of 

 slightly recurved acute cones arranged along the sides of each 

 jaw in two nearly parallel or converging lines, continued so as 

 to meet in the front of the mouth. As we advance higher in the 

 scale of life these teeth are gradually differentiated, those in 

 front becoming chisel-shaped for cutting, those at the angle 

 being greatly elongated and strengthened for seizing ; those 

 next in order shortened and made more blade-like for splitting 

 or breaking ; and those behind blunted on their crowns and 

 gradually transformed into grinders. The differentiation between 

 these teeth at first is not very marked, the intermediate teeth 

 often partaking of an intermediate character ; but as the teeth 

 become more highly specialized the intermediate forms are 

 eliminated, the jaws shortened, and the teeth reduced to four or 

 fewer definite classes of teeth, each being often very distinct 

 from its neighbours. These four classes are known as incisors, 

 canines, premolars, and molars. 



In the land Carnivora these teeth are all more or less 

 marked. They have six incisors above and below with one or 

 two exceptions, two canines above and below of the highest 

 type, and behind these, premolars and molars of at least two 



