Clifton College Scientific Society. 53 



so are classed according to their teeth and scales. Distinguished 

 by these, M. Agassiz has divided fishes into four great classes : — 



1. Placoid. 



2. Ganoid. 



3. Ctenoid. 



4. Cycloid. 



The placoid fishes have the skin covered irregularly with 

 enamelled plates, sometimes of large size, but frequently in small 

 points (as the shagreen of sharks). 



Ganoidians are covered with brilliantly enamelled plates of an 

 angular form, composed of horn or bone ; their structure is 

 identical with that of the teeth. 



Ctenoid scales are plates, which are toothed or pectinated like a 

 comb on their posterior margin. By being superimposed on each 

 other, so that the lowermost edges always extend beyond the 

 uppermost, their numerous points render them very harsh to 

 touch. 



In Cycloid fishes the scales are composed of simple laminae 

 without enamel, with sfnooth borders ; but their external surface 

 is often ornamented with markings. 



In most cases the scales are imbricated, by which their true 

 form is often concealed ; if not, they resemble mere points scarcely 

 visible to the naked eye. There are few genera destitute of scales. 

 Teeth of fishes also serve to determine the classes. Of all the 

 durable parts of animals found in a fossil condition, the teeth of 

 fishes are certainly the most numerous, presenting most varied 

 forms and structure, composition, arrangement, and attachment ; 

 yet without the aid of other parts of the skeleton, it would often 

 be hard to decide whether an unknown tooth belonged to a reptile 

 or a fish. 



In spite of the numberless modifications in their forms, teeth 

 can be reduced to four principal types : — 



1. Conical ; 2. Flattened ; 3. Prismatic ; 4. Cylindrical — all 

 more or less varied. Of the conical teeth, many are long and 

 striated at the base, like those of reptiles. The teeth are com- 

 posed of a dense osseous material, of a tubular structure, called 

 dentine ; which, in many cases, forms on the external surface a 

 hard texture, very glossy, resembling enamel. The essential 

 character of their organisation is to have a medullary cavity, filled 

 with a plexus of blood-vessels and nerves, from which minute 

 tubes, forming the dentine, radiate. 



The vertebrae of fishes are two hollow cones, resembling an hour- 

 glass in shape ; in life, the space between two bones is filled up 

 with a gelatinous fluid. 



