TEETH AND FOOD 127 



more typical Guitar-fishes (Rhinobatus) the mouth forms a 

 straight horizontal slit. In some of the Rays and Skates, of 

 which the common Thornback Ray (Raia clavata) will serve as 

 an example, the teeth are actually different in the two sexes, 

 those of the male being pointed and those of the female flat. 

 It has been argued that this difference in the form of the teeth 

 indicates a corresponding difference in the food or some sort 

 of co-operation between the sexes in procuring it, but this is 

 purely supposition and is not supported by any evidence. 



The dentition of the Eagle ^div {Myliobatis) is very specialised, 

 the teeth being quite flat and arranged like paving stones in the 

 form of a mosaic work, those in the centre of the jaws having 

 the form of long hexagonal bars, and those at the sides being 

 much smaller but also six-sided (Fig. 5 id). In the large Spotted 

 Eagle Ray (Aetobatis) these side teeth are wanting, and the 

 dentition consists of a single row of long bars arranged one 

 behind the other from before backwards in each jaw. The food 

 seems to consist almost entirely of oysters and clams, and the 

 crushing power of the jaws is truly remarkable. "I have found 

 in these Rays," writes Mr. Coles, "clams which with their 

 shells on must have weighed more than three pounds, and to 

 crack which a pressure of perhaps a thousand pounds would 

 be required." In the Sea Devils {Mobulidae), on the other hand, 

 with their fish diet, the teeth are very small, numerous, and 

 have the form of flat tubercles. 



The Chimaeras (Holocephali) , although alhed to the Sharks 

 and Rays, present a totally different dentition. Instead of the 

 ordinary teeth, the jaws are armed with three pairs, two above 

 and one below, of large flat plates, studded with hardened 

 points or "tritors" (Fig. 56A). In one or two species these 

 points are absent, and the tooth plates bear a marked re- 

 semblance to the horny "beaks" of turtles. With such a 

 speciahsed dentition one might reasonably expect the food to 

 be of a very definite nature, but actually the diet is a very 

 mixed one and ranges from seaweeds to other fishes, including 

 also worms, echinoderms, molluscs, and crustaceans.^ They 

 are themselves preyed upon by other fishes, adult Chimaeras 

 being found in the stomachs of Greenland Sharks, and the 

 young are eaten in large numbers by the Cod and its allies. 



In the Selachians teeth are developed only in the jaws, but 

 in the Bony Fishes they may be present on the tongue, on the 

 roof of the mouth, or in the throat. The arrangement may be 

 quite irregular, or they may be placed in one or more regular 



