STRUCTURE OF CERTAIN PAL/EOZOIC DIPNOI. 
177 
lies in the form of the hind border. In the " dentary " of Ceratodus this 
border is thin and papery like the rest of the bone. In the Sagenodus 
" dentary ■" it is widened abruptly, and presents a deep, polished face toward 
the gular space (fig. 15, D, p. 180). This part of the jaw is, in fact, 
finished oft' in almost- precisely the same way as the corresponding part in 
Dipterus, and, as we show later, for the same purpose : to form a bearing for 
gular plates. 
All the elements of the jaw are as variable as the teeth have long been 
known to be. Fig. 12 B, p. 178, shows that some jaws are deeper than that 
figured above it at C, and certain angulars in the Atthey Collection would fit 
Fio. 11. 
Supposed Quadrates, x about f. 
A, B. Supposed to be of Ctenodus. C, D. Sagenodus. (A and B are opposite sides 
of the same specimen : C and D are opposite sides of two different specimens.) 
a jaw of this deeper form. Other angulars suggest an even shallower jaw 
than C. Well-preserved angulars show that both the upper and lower borders 
of the bone were rolled inwards very considerably ; the lower part towards 
the middle jsf its length formed a sort of flopr inside (fig. 12, F, p. 178), 
and the upper edge bears a bracket for the support of the back of the tooth- 
plate, as was noticed by Atthey (1877, p. 228). Very small angulars, of 
3 or 4 cm. in length, not having been seriously crushed, exhibit this inward 
12* 
