340 ORDERS OF FISHES. 



resemble those of the Hybodonts,but the back teeth are adapted 

 for crushing shell-fish. Each of these crushing teeth has an elon- 

 gated form, with a rounded 

 surface, which is covered 

 with fine transverse striae 

 proceeding from a central 

 longitudinal line. From their 

 general form, colour, and 

 Fig. 3 oo. Tooth of Acrodus tiobiiis. Lias. striation, they are common- 

 ly called "fossil leeches" by 



the quarrymen. As in the case biHybodus, the species viAcrodus 

 are exclusively 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 preceding, the 

 species of Strophodus range from the Trias to the Chalk. 



In the genus Ptychodus, 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 Cestraphori have been described; 

 but the affinities of most of these are doubtful. At the present 

 day the family is represented by the single living species, the 

 Port Jackson Shark (Cestracion Philippi). 



b. SelachiL 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 con- 

 sist of teeth. 



In the Jurassic series are found teeth of Notidanus and 

 Sphmodus. In the Cretaceous Rocks are numerous teeth, 

 referred to the genera Corax, Galeocerdo^ Otodus, Lamna, 

 Oxyrhina, and Odontaspis, all of which continue to be repre- 

 sented in the Tertiary deposits. The teeth of Carcharodon 

 (fig. 303) also occur in the Cretaceous series, but the genus is 

 mainly Tertiary. The teeth of Carcharodon are triangular, 



