DEVONIAN FISHES OF IOWA 95 



Class PISCES. 



Among the more distinctive characters of Fishes, as com- . 

 pared with all higher Craniates, may be mentioned the following : 



Animals of this class are jaw-bearing chordates of fresh-water 

 or marine habitat, which in their shape, locomotor and respira- 

 tory organs are completely adapted for an aqnatic life. In all 

 forms breathing is normally by gills, though in Dipnoans (or 

 Lung-fishes) these merely assist the lungs. The gills are derived 

 from the walls of the branchial clefts, and are supported by a 

 series of branchial arches. The principal organ of locomotion 

 is the powerful tail ; in addition, however, there are paired fins, 

 pectoral and pelvic, corresponding to the fore and hind limbs of 

 the terrestrial Craniata, and possessing a supporting carti- 

 laginous or bony skeleton (''ichthyopterygium") which cannot 

 readily be compared with the limb-skeleton of the latter. Fishes 

 also possess a system of median fins, supported by a special 

 skeleton of their own. An exoskeleton of dermal spines or denti- 

 cles, scales or bony plates, is usually present. Except in one 

 group, the Dipneusti, the heart has but one auricle, and receives 

 only venous blood, which it forces, first, through the blood-ves- 

 sels of the gills, and thence, as arterial blood, through the ves- 

 sels of the body generally. An air-bladder is frequently present, 

 and serves as an hydrostatic organ or float, but in a few cases it 

 may act as a lung, and helps the gills in the work of respiration. 

 The paired olfactory organs rarely communicate with the oral 

 cavity by internal nostrils. Peculiar cutaneous sense-organs are 

 disposed in linear tracts along the sides of the body (lateral line 

 sensory organs) and on the head, and appear to be specially 

 associated with a life in water. 



Subclass ELASMOBRANCHII. 



Sharks and rays, both ancient and modern, agree in having the 

 exoskeleton composed of a more or less uniform investment of 

 dermal papillae or denticles called "shagreen". The term pla- 

 coicl is also sometimes applied to this type of cutaneous cover- 

 ing. The endoskeleton is wholly cartilaginous or partially calci- 

 fied, and the skull is without either cartilage- or membrane- 

 bones. The mode of attachment of the primitive upper jaw 



