282 PALAEONTOLOGY. 



8fch Family. The AcuoLEPisiDyE Include the great 

 sauroid fishes, with heterocercal tails, and large pointed 

 conical teeth alternating with smaller ones. The genera 

 SauricJitJiys and ^MegalichtJiys were the tyrant monsters of the 

 waters they inhabited. Five of the twelve genera it includes 

 appeared for the first time in the Devonian group ; they 

 attained their greatest development in the carboniferous 

 stage, and became extinct with the triasic. 



9th Family. The POLTPTEEID^ Are sauroid fishes, 

 with hornocercal tails. Of the sixteen genera of this family, 

 fourteen are extinct, and two are still living. They were 

 created at the commencement of the oolitic period, after 

 the last heterocercal sauroids had become extinct. They are 

 most abundant in the Oxford and upper stages of the oolitic 

 period, and are found in the chalk. The Polypterus of the 

 Nile, and the Lepidosteus, or bony pike, of the American 

 lakes and rivers, are the living types. 



10th Family. The DIPTERID^: resemble the ACROLEPI- 

 SID.E, in the form of the tail, and of their scaly armour ; but 

 they possess two dorsal and two ventral fins. We find two 

 genera the Osteolepis, in the Devonian stage : and the 

 Dipterus, in the carboniferous. 



llth Family. The ACANTHODIDJE have the tail hetero- 

 cercal ; the teeth unequal, and the body covered with 

 microscopic scales. The four genera of this group are 

 Devonian one passes into the carboniferous stage, but the 

 family became extinct at the close of the palaeozoic epoch. 



12th Family. The PAL^ONSICID^; comprehends the 

 Lepidoid fishes with heterocercal tails ; they have small 

 teeth arranged on many lines in the interior of the mouth ; 

 the scales are rhomboidal in form, and are arranged in 

 parallel lines on the body ; they have one dorsal and one 

 ventral fin. "We know six genera ; five commenced with the 

 carboniferous stage, of which three passed into the triasic, 

 and one is special to the Oxford oolite. 



13th Family. The LEPIDOTIDJS embraces the Lepidoid 

 fishes with homocercal tails. The ten genera which compose it 

 are all extinct ; seven genera are found in the lias. It attained 

 its maximum development of species in the Oxford stage. 

 In the cretaceous strata there are three, and in the inferior 

 tertiary, only one, the Lepidotus. 



