XIX DIPNEUSTI 507 
America), and Sagenodus (Carboniferous of Great Britain and 
Lower Permian of Bohemia) belong to the same family. 
Fam. 2. Uronemidae.— Body fusiform. Dentition non- 
etenodont, consisting of patches of distinct rounded denticles with 
a row of basally-confluent conical denticles along the outer 
margin of each. Scales thin. All the median fins are con- 
tinuous. Tail apparently diphycercal. Cranial dermal bones as 
in Dipterus. Uronemus* (Lower Carboniferous of Scotland), and 
perhaps Conchopoma® (Lower Permian of Prussia), are the only 
known genera. 
The two remaining familes possess certain features which cannot 
be affirmed to have existed in their extinct allies. Thus, both agree 
in exhibiting those striking and, so far as Fishes are concerned, 
unique modifications of the air-bladder and vascular system, and the 
olfactory organs,’ which are more or less closely associated with 
air-breathing habits and indicate a marked convergence towards 
the Amphibia. Side by side with such indications of advancing 
specialisation in certain directions, ample evidence of a remote 
ancestry is to be seen in such primitive features as the presence 
of a spiral valve and a multi-valvular conus arteriosus, and in the 
short and simple alimentary canal. Of other points of agreement 
mention may be made of the absence of jugular plates, the 
presence of vomerine teeth, the continuity of all the median 
fins, and the apparently diphycercal but probably gephyrocercal 
character of the tail. 
Fam. 3. Ceratodontidae.—Body elongated and compressed. 
Scales large, thin, non-ganoid, and partially enclosed in dermal 
pouches. Paired fins biserial. Chondrocranium complete. Der- 
mal bones wholly devoid of ganoin, reduced in number but 
increased in size. Circumorbital bones present. Dental plates 
oval, crescentic or triangular, traversed by several radiating 
enamelled ridges, terminating in smooth or feebly denticulated 
biting margins. Lower jaw with a small toothless dentary on 
each side. The hyoid arch includes a small hyomandibular and a 
hypo-hyal in addition to a cerato-hyal. Branchial arches five in 
number and bisegmented. The gills exhibit little evidence of 
1 Traquair, Journ. Roy. Geol. Soc. Ireland (N.S.), iii. 1873, p. 41; Proc. Roy. 
Soc. Edinb. xvii. 1890, p. 393. 
2 Kner, SB. k. Akad. Wiss. Math.-Naturw. Cl. lvii. Pt. ii. 1868, p. 279. 
3 See Chaps. XI. XII. and XIV. 
