Teleostomi. 



97 



Homosteus (Hugh Miller's " Asterolepis of Stromness ") is 

 a very similar fish, with toothless jaws and the eyes within the 

 head- shield. Plaster casts of the shields of Homosteus Milleri 



FIG. 134. Coccosteus decipiens, Ag. ; Lower Old Red Sandstone, Scotland. 



from the Caithness flagstones are exhibited on the pillar 



between Wall-cases Nos. 4 and 5. There are fragments of Wall-case, 



other species from the Devonian of Livonia, Russia. Heterosteus " ' 



is a gigantic fish from the Devonian of Livonia, with a great 



bony process from the body-shield extending forwards on each 



side of the head. 



Dinichthys is a still larger Arthrodiran from the Upper 

 Devonian of Ohio, U.S.A. Its dentition (Fig. 135) much 

 resembles that of the recent Protopterus. 



<- 



js^/v^i. 



FIG. 135. Jaws of DinichtJtys ; Devonian, North America. 



Sub-class IV. TELEOSTOMI. 



These are fishes with a bony armour or bony skeleton, or 

 both ; with the margin of the mouth completed by membrane- 

 bones ; with the more or less ossified cartilages of the upper 

 jaw suspended from the skull by the upper part of the hyoid 

 arch (hyomandibular) ; and with a bony operculum covering 

 the gill-cavity. The name of the sub-class Teleostomi (com- 

 plete-mouth) refers to the ossification of the margin of the 

 jaws. 



Nearly all the Devonian representatives of this sub-class 

 have lobate paired fins fringed with dermal rays, and are thus 

 named CROSSOPTERYGII (fringe-fins). A single Devonian genus, 

 Cheirolepis, belongs to a higher order which began to replace 

 the Crossopterygians in the Carboniferous period, and which 

 is named ACTINOPTP^RYGIT (ray-fins) because here the lobe is 

 insignificant and the enlarged dermal rays support almost c>r 

 quite the whole of each paired fin. 



(1876) 8 



