196 FISHES. 



Polypterus, with which some of the genera like Diplopierus 

 singularly agree in the form and armature of the head, 

 the lepidosis of the body, the lobate pectoral fins, and 

 the termination of the vertebral column. Other genera, as 

 HoloptycMus, have cycloid scales ; many have two dorsal fins 

 {Holoptychius), and, instead of branchiostegals, jugular scutes ; 

 others one long dorsal confluent with the caudal {Phanero- 

 pleuron). 



In the second type the principal characters of the Dipnoi 

 are manifest, and some of them, for example Dipterus, Palce- 

 daphus, Holodus, approach so closely the Dipnoi which still 

 survive, that the differences existing between them warrant a 

 separation into families only. 



Devonian fishes are frequently found under peculiar cir- 

 cumstances, enclosed in the so-called nodules. These bodies 

 are elliptical flattened pebbles, which have resisted the action 

 of water in consequence of their greater hardness, whilst 

 the surrounding rock has been reduced to detritus by that 

 agency. Their greater density is due to the dispersion in 

 their substance of the fat of the animal which decomposed in 

 them. Frequently, on cleaving one of these nodules with 

 the stroke of the hammer, a fish is found embedded in the 

 centre. At certain localities of the Devonian, fossil fishes are 

 so abundant that the whole of the stratum is'affected by the 

 decomposing remains emitting a peculiar smell when newly 

 opened, and acquiring a density and durability not possessed 

 by strata without fishes. The flagstones of Caithness are a 

 remarkable instance of this. 



The fish-remains of the Carboniferous formation show a 

 great similarity to those of the preceding. They occur 

 throughout the series, but are very irreg^ilarly distributed, 

 being extremely scarce in some countries, whilst in others 

 entire beds (the so-called bone-beds) are composed of ichthyo- 



