502 THE MOST ANCIENT FISHES. 



living Ceratodus. The sharks are one of the most ancient groups of 

 fishes ; at least one of the oldest known fishes from the Ludlow bone 

 bed, Onchus, is a shark. It is succeeded in the Devonian rocks by Dime- 



Heterocercal. Homocercal. 



Fig. 1 36. -Fish tails. 



racanthus and H&macanthus. In the Carboniferous rocks succeed Ora- 

 canthus, Gyracanthzts, Tristychius, Astroptychius, Ptychacanthus, and 

 Sphenacanthus. In the Coal-measures are found such types as Clada- 

 canthus, Leptacanthus, and Gyropristis. Leptacanthus ranges from the 

 Coal to the Oolite. The Trias yields some peculiar genera like 

 Nemacanthus and Liacanthus ; while Aster acanthus, Myriacanthus, 

 and Pristacanthus are known from the Oolites. These are a few of 

 the genera founded upon fish spines. Several existing genera of 

 sharks are actually or closely represented in the strata, thus Corax, 

 closely allied to the Blue Shark, is Cretaceous, and Tertiary. Car- 

 charias is found in the Chalk, in which Hemipristis and Galeocerdo 

 occur. The Lamna family, which comprises the living porbeagles, 

 first appears with Carcharopsis in the Carboniferous period. Carcha- 

 rodon, which yields teeth in the Crag five inches long, is not known 

 to attain a greater length than forty feet, though teeth as large as the 

 crag teeth occur at the bottom of the Pacific : it is recorded from the 

 Maestricht beds and ranges through the Tertiary. The genus Lamna, 

 which includes Oxyrliina, is abundant in the Cretaceous and dates back 

 to the Wealden. Among allied fossil types are Splienodus from the 

 Oolites ; Gomphodus and Ancistrodon from the Chalk. The grey 

 sharks, Notidanus, are represented in the Brown Jura, and in this 

 country are found in the Upper Greensand and London Clay. JEllopos 

 is found in the Solenhofen Slate. The dog-fishes are well represented 

 in the Secondary strata, and are known as Scylliodus, Palceoscyllium, 

 and Pristiurus, which is a well-known European genus. The Hybo- 

 donts are all extinct. Cladodus, in the Devonian and Carboniferous, 

 is the oldest representative of the group. Hybodus is said to occur in 

 the Carboniferous and in the Tertiary, but is characteristic of the 

 Secondary Rocks. The Cestraciodonts appear with Ctenoptyclnus 

 in the Devonian, Psammodus, Cocliliodus, and Polyrhizodus in the 

 Carboniferous ; Strophodus and Acrodus in the Oolites ; and Ptychodus 

 in the Chalk. The picked-dogs are represented in the Lias by Palceo- 

 spinax, but the genus Spinax does not date back further than the 

 Secondary Rocks. 



OrtUacanthus of the Carboniferous may have been a monk fish ; 



