3590 The Zoologist — July, 1873. 



but possessing neither lacunae nor vascular canals. This is a very 

 common form amongst fishes. 



In the common wrasse or connor of our shores we find this 

 harder external layer developed inwards, to the extinction of all 

 the vaso-dentine. The tooth is entirely composed of hard, very 

 finely tubular dentine, but this construction seems to interfere with 

 the vital connection of the tooth with the living jaw, as there is a 

 provision for a constant succession of teeth from below. 



Indeed in fishes generally there seem to be few examples of 

 teeth being implanted by fangs in a socket, and also there seems 

 to be no great permanency of connection between the teeth and 

 their possessors : there is generally a provision for a constant suc- 

 cession either from behind forwards, as in the sharks and rays, or 

 from below upwards, as in the wrasse, already mentioned, or as in 

 the angler, where they rise up between the old ones, which fall 

 away. The law seems to be that of irrelative repetition. There is 

 no instance amongst fishes of such a continuously growing tooth 

 as we find in the wombat, which if it were a probable structure to 

 occur from separate origins we might expect, since there is much 

 greater variety of form and number of species for it to occur in 

 among fishes than mammals. The dental apparatus of the parrot- 

 fish is one of the nearest examples to the teeth of rodents in 

 function that I can find ; that of the Lepidosiren looks something 

 like in section, but I do not know sufficient of the habits of this 

 animal to say anything of the functions of its curious-looking jaw. 

 Amongst reptiles the same law of constant succession of teeth holds 

 good, which looks as if there was the same difficulty of retaining 

 the teeth permanently, but when we arrive at Mammalia we find at 

 most only one change of teeth, and this apparently in order to 

 accommodate the adultx animal with a larger set than would have 

 been convenient for its young state. 



The peculiarities of structure which perform this apparently 

 difficult feat are these: — the part of the tooth most exposed to 

 wear is protected by enamel, which is extremely hard, and, so far 

 as we know, entirely devoid of life; below this, and immediately 

 surrounding a single vascular permanent calcigerous pulp, is the 

 body of the tooth, formed of dentine, which is traversed by an 

 immense number of fine tubes passing from the pulp to the cir- 

 cumference. These tubes being occupied by fine processes of the 

 calcigerous cells, which, as we have before seen in the development 



