The Crossopterygii O25 
We may divide the Teleostomes, or true fishes, into three 
subclasses: the Crossopterygii, or fringe-fins; the Dipneusti, or 
lung-fishes; Actinopteri, or ray-fins, including the Ganoidei and 
the Teleoste:, or bony fishes. Of these many recent writers are 
disposed to consider the Crossopterygit 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 
Teleostet (constituting the Actinoptert) 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 called after Huxley, Crossop- 
terygit (kpoooos, fringe; mrepvé, 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 Ichthyotom:t among sharks, and with 
the Dipneustiz. From the latter it differs 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 cesophagus. 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 
