DIPXEUSTI 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- 

 ctenodont, 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 l (Lower Carboniferous of Scotland), and 

 perhaps Conchopoma* (Lower Permian of Prussia), are the only 

 known genera. 



The two remaining families 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, 3 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. Jf r iss. Math.-Natunc. CL Ivii. Pt. ii. 1868, p. 279. 



3 See Chaps. XI. XII. and XIV. 



