ORDERS OF FISHES. 161 



has an elongated form, with a rounded surface, which is 

 covered with iine transverse strise proceeding from a central 

 longitudinal line. From 



their general form, colour, ^_____™.-as«^=^^^^v^armr"-3irT 

 and striation, they are com- 

 monly called "fossil leeches" 

 by the quarrymen. As in 



the case of HyloduS, the Fig. 526.-Tootli of ,4crod«s noWUs. Lias. 



species of Acrodus are ex- 

 clusively Mesozoic, ranging from the Trias to the Chalk. 



The teeth of Strophodus are elongated, very similar to 

 those of Acrodus in their general form, but truncated at both 

 ends, and having their surface reticulated. Like the preced- 

 ing, the species of Strophodus range from the Trias to the 

 Chalk. 



In the genus Ftycliodus, lastly, the teeth are more or less 

 quadrate in form, and the summit of the crown of the tooth 

 is thrown into parallel transverse folds, ridges, or plications, 

 surrounded by a granulated area. All the species of this 

 genus are Cretaceous. 



A few Tertiary forms of the Ccstraphori have been de- 

 scribed ; but the afJ&nities of most of these are doubtful. At 

 the present day the family is represented only by the few 

 species of the genus Cestracion. 



h. Selachii. — This group comprises the Dog-fishes and 

 Sharks, characterised by the elongated, not rhomboidal, form 

 of the body, and by the lateral position of the gill-slits on 

 the sides of the neck. The teeth are sharp and conical, and 

 are arranged in several rows, of which the outermost alone is 

 employed, the inner ones serving to replace the former when 

 worn out. 



This family attains its maximum at the present day, and 

 its earliest authentic representatives appear in the Jurassic 

 period. Some Palaeozoic fossils, however, have been, with 

 more or less probability, referred to Sharks, or placed in the 

 neighbourhood of the living Monk-fishes {Squatina). With 

 the exception of occasional vertebrae, all the known remains 

 of Selachians consist of teeth. 



In the Jurassic series are found teeth of Notidanus and 

 VOL. II. L 



