76 GUIDE TO THE FOSSIL REPTILES, AMPHIBIANS, FISHES. 
Wall-case 
3 . 
Table-case 
9 . 
W all-case 
5 . 
Table-case 
10 . 
Wall -case 
5 . 
Table-case 
10 . 
the existing Callorliynchus in the shape of its snout, but is 
peculiar in having a supplementary chisel-shaped tooth 
in front of the lower jaw. The still-surviving family of 
Chimseridse is first represented by teeth of G-anodus and 
Ischyodus in the Lower Oolites (Table-case 9), the latter genus 
also ranging to the Upper Cretaceous. Good skeletons of 
Ischyodus are known from the Lithographic Stone of Bavaria, 
and part of one is exhibited in Wall-case 3. Some of 
the teeth of Ischyodus and of the Cretaceous and Eocene 
Edaphodon (Eig. 76) indicate species which must have been 
gigantic compared with any Chimseroid now living. Chimazra 
itself dates back at least to the Pliocene. 
Sub-class III.— Dipnoi. 
The first ordinary fishes with a gill-cover and bony 
tissue in their skeletons are found in the Middle Old Bed 
Sandstone. They have enamelled bony scales and external 
head-bones, but very little hardening of the internal cartila¬ 
ginous skeleton. Their paired fins are paddle-shaped, with 
an internal skeleton of cartilage; and their tail is always 
diphycereal or heterocercal (see p. 61). Among these fishes 
one group is remarkable for the fusion of the upper jaw with 
the skull, as in Chimseroids and land vertebrates; and this 
peculiarity is combined with others suggesting that the 
group in question is connected in some way with the ancestors 
of the land vertebrates which must have been living in the 
Devonian period. The survivors of this group are provided 
not only with the ordinary gills but also with an air-bladder 
so modified that it can be used as a lung. The Sub-class 
they represent is therefore known as that of the Dipnoi 
(“ double-breathers ”). 
Order I.— SIRENOIDEI. 
The living Dipnoi are confined to the widely-separated 
fresh-waters of South America (Lepidosiren), Africa (Prom¬ 
pter us), and Australia (Geratodus). In past geological times 
the Order to which they belong was cosmopolitan. The 
earliest known genus is Dipterus (Figs. 77, 78, 1), of which 
good specimens from the Caithness flagstones are shown in 
Table-case 10. It is characterised by two dorsal fins (hence 
