The Crossopterygii 599 



We may divide the Teleostomes, or true fishes, into three 

 subclasses: the Crossopterygii, or frmge-fins; the Dipneusti, or 

 lung-fishes; Actinopteri, or ray-fins, including the Ganoidei and 

 the Teleostei, or bony fishes. Of these many recent writers are 

 disposed to consider the Crossopterygii as most primitive, and 

 to derive from it by separate lines each of the remaining sub- 

 classes, as well as the higher vertebrates. The Ganoidei and 

 Teleostei (constituting the Actinopteri) are very closely related, 

 the ancient group passing by almost imperceptible degrees into 

 the modern group of bony fishes. 



Subclass Crossopterygii. — The earliest Teleostomes known 

 belong to the subclass or group caUed after Huxley, Crossop- 

 terygii {Kpoacro;, fringe; nrepvS, fin). A prominent character of 

 the group lies in the retention of the jointed pectoral fin or archip- 

 terygium, its axis fringed by a series of soft rays. This char- 

 acter it shares with the IclitJiyotomi among sharks, and with 

 the Dipneusti. From the latter it dift'ers in the hyostylic cra- 

 nium, the lower jaw being suspended from the hyomandibular, 

 and by the presence of distinct premaxillary and maxillary 

 elements in the upper jaw. In these characters it agrees with 

 the ordinary fishes. In the living Crossopterygians the air- 

 bladder is lung-like, attached by a duct to the ventral side 

 of the oesophagus. The lung-sac, though specialized in struc- 

 ture, is simple, not cellular as in the Dipnoans. The skeleton 

 is more or less perfectly ossified. Outside the cartilaginous 

 skull is a bony coat of mail. The skin is covered with firm 

 scales or bony plates, the tail is diphycercal, straight, and end- 

 ing in a point, the shoulder-girdle attached to the cranium is 

 cartilaginous but overlaid with bony plates, and the branchios- 

 tigals are represented by a pair of gular plates. 



In the single family represented among living fishes the 

 heart has a muscular arterial bulb with many series of valves 

 on its inner edge, and the large air-bladder is divided into two 

 lobes, having the functions of a lung, though not cellular as in 

 the lung-fishes. 



The fossil types are very closely allied to the lung-fishes, 

 and the two groups have no doubt a common origin in Silurian 

 times. It is now usually considered that the Crossopterygian 

 is more primitive than the lung-fish, though at the same time 



