Or THE TEETH. 139 



and *; and in some animals they are placed in the stomach, 



as in the water- insect or crab, <fce. 



Those in the month may be divided into two situations : — First, all 

 those forming two rows in each jaw (i. e. one row in the right, and 

 another in the left jaw), and opposed by similar rows in the opposite 

 jaws ; secondly, where the teeth are placed on other parts, as the 

 tongue. The first situation admits of divisions, as where those rows 

 are single, as in the quadruped and amphibia ; [or where] they are 

 double, triple, &c. rows, as in many fish, where the fcnir rows mentioned 

 are composed of a vast number of rows of teeth. 



They may be classed according to their uses, which I shall at present 

 reckon four, viz. holders or retainers, which may be called killers, 

 dividers, crackers, and grinders ; the two last of which may be thought 

 the same. 



The dividers are always more external than the grinders. Some 

 dividers are always external, others are some way within, some more, 

 as in the — (nereis ?) ; some less, as in the snail. Some of the grinders 

 are as far forward as the dividers will allow them, as in those which 

 have mouths filled with both kinds, as in most of the more perfect 

 animals ; but in many those grinders are placed in the stomach, but 

 then those have their dividers wholly external [as in the lobster]. 

 Teeth are commonly fixed in or upon some bone, which [bones] are 

 commonly the jaws of the animal ; but this is not always the case ; in 

 the lamprey there is no jaw-bone. 



Some teeth grow to a given size, and then become stationary, as in 

 most animals, viz. the human, &c, some of which teeth last through life ; 

 others, of the same animal, are thrown off at given ages, called shedding 

 of the teeth, and are again supplied by others, which last through life. 

 In some other animals there is a regular succession of teeth, by the 

 falling off or destruction of the teeth, and new ones continually growing 

 and gradually coming into use ; the new teeth being always a pro- 

 portional size longer than the old ; the jaws of which [animals] follow the 

 same course, so that there is a regular succession of jaw and teeth 

 growing: this is the case in many fish, as in all the ray-kind. In 

 others there is a succession of young teeth growing at the basis of the 

 old, or rather within the old, so that the old (tooth) drops out like a 

 conical case, and the young one is uncovered [Crocodile]. Probably 

 the young tooth grows on the same pulp, so that these teeth never 



1 [Q.uaere, Myxine, Nereis? The term Reptiles is commonly applied by Mr. 

 Hunter to the Vermes of Linmeus ; whilst the Beptilia of the modern zoologists he 

 usually denominates Amphibia or Tricoilia.] 



