Subclass Dipneusti, or Lung-fishes 237 
developed bony plates made of ossified skin and not corre- 
sponding with the membrane-bones of higher fishes. The fish- 
like membrane-bones, opercles, branchiostegals, etc., are not 
yet differentiated. The teeth have the form of grinding-plates 
on the pterygoid areas of the palate, being distinctly shark-like 
in structure. The paired fins are developed as archipterygia, 
often without rays, and the pelvic arch consists of a single 
cartilage, the two sides symmetrical and connected in front. 
There is but one external gill-opening leading to the gill-arches, 
which, as in ordinary fishes, are fringe-like, attached at one 
end. In the young, as with the embryo shark, there is a bushy 
external gill, which looks not unlike the archipterygium pec- 
toral fin itself, although its rays are of different texture. In 
early forms, as in the Ganoids, the scales were bony and enam- 
eled, but in some recent forms deep sunken in the skin. The 
claspers have disappeared, the nostrils, as in the frog, open 
into the pharynx, the heart is three-chambered, the arterial 
bulb with many valves, and the cellular structure of the skin 
and of other tissues is essentially as in the Amphibian. 
The developed lung, fitted for breathing air, which seems 
the most important of all these characters, can, of course, be 
traced only in the recent forms, although its existence in all 
others can be safely predicated. Besides the development 
of the lung we may notice the gradual forward movement 
of the shoulder-girdle, which in most of the Teleostomous 
fishes is attached to the head. In bony fishes generally 
there is no distinct neck, as the post-temporal, the highest 
bone of the shoulder-girdle, is articulated directly with the 
skull. In some specialized forms (Balistes, Tetraodon) it is 
even immovably fused with it. In a few groups (Apodes, 
Opisthomi, Heteromi, etc.) this connection ancestrally possessed 
is lost through atrophy and the slipping backward of the 
shoulder-girdle leaves again a distinct neck. In the Amphib- 
ians and all higher vertebrates the shoulder-girdle is dis- 
tinct from the skull, and the possession of a flexible neck is 
an important feature of their structure. In all these higher 
forms the posterior limbs remain abdominal, as in the sharks 
and the primitive and soft-rayed fishes generally. In these 
the pelvis or pelvic elements are attached toward the middle 
