388 NATURAL SCIENCE. j UNE , 



closely packed as to form a dental mosaic or pavement, e.g., Rhinoptera, 

 and the eagle-ray, Myliobatis. The central tooth -plates are, in such 

 cases, usually larger than the marginal plates, although this inequality 

 is not apparent in the young. In Actobatis the central plates alone 

 are present. Small denticles frequently occur embedded in the 

 general mucous membrane of the mouth, and over the pharyngeal 

 borders of the branchial arches, e.g., Galeus and Cestvacion. These 

 denticles, being less specialised than the teeth of the jaw, bear a much 

 closer resemblance to the placoid spines of the integument than do 

 the teeth. In the basking-shark (Selache) long comb-like gill-rakers, 

 said to be composed of dentine, project inwards from the branchial 

 arches and act as strainers, preventing the escape, through the gill- 

 clefts, of the small animals taken into the mouth as food. Very 

 similar gill-rakers are found in the ganoid fish, Polyodon, and in some 

 teleosteans, e.g., the shad (Clupea alosa). In the saw-fish (Pvistis), the 

 prenasal rostrum is greatly elongated and armed with a series of 

 strong teeth along each margin, in the form of a two-edged saw. The 

 teeth are arranged at regular intervals, and are implanted in distinct 

 sockets. They are not shed, but continue to grow from persistent 

 pulps, and, if lost accidentally, they are not replaced. 



No great development of teeth is to be found in the cartilaginous 

 ganoids. The mouth of the sturgeon has the form of a protrusible 

 suctorial tube, devoid of teeth ; but true teeth are present in the larva 

 (Fig. 1 8). In Polyodon the jaws are armed with numerous minute teeth, 

 embedded in the mucous membrane, but not connected with the 

 underlying bone. In the Dipnoi there are two pairs of large grinding 

 teeth, one pair in the mandible (Fig. 19, s) anchylosed to the splenial 

 bones, and the other pair (p) borne by the palato-pterygoid bones in 

 the roof of the mouth. There is also a small pair, known as the 

 vomerine teeth (v), situated in front of the palatine pair. These are 

 not in relation with any of the bones of the skull, but are loosely fixed 

 to the under surface of the ethmoidal cartilage. In Cevatodus the inner 

 border of the large teeth, both upper and lower, is convex, and the 

 outer edge is, in the living species (C. forstcvi), marked by five deep 

 notches. The two vomerine teeth have the form of plates in contact 

 anteriorly, but diverging behind. The large teeth of Cevatodus were 

 known in the fossil state long before the discovery of the existing 

 species. In the ancient forms, Ctenodus, Diptevus, etc., the general 

 build of the teeth is the same as in Cevatodus, but the complexity is 

 greater. In Pvotoptevus there are only two notches in each of the large 

 teeth, and the vomerine teeth are slender and conical in shape. The 

 palato-pterygoid teeth of the^ right and left sides are in contact in the 

 median line, as are also the mandibular teeth ; but this is not so in 

 Cevatodus. The mandibular teeth are situated more anteriorly than 

 in Cevatodus. The dentition of Chimava bears a striking resem- 

 blance to that of the Dipnoi. The external borders of the teeth are 

 not notched, but the mandibular teeth show a pair of broad shallow 



