STRUCTURE OF CERTAIN TAL^OZOIC DIPNOI. 213 
interfrontal and internasal bones demands descent from a form not later than 
D. platycephalus, and cannot Iiave arisen from tbe much more reduced skulls 
of Pentlandia, Scaumenaeia, and Phaneropleuron. 
The Sagenodus skull-roof can readily be derived from that of Ctenodus. 
In its dentition Sagenodus is much more primitive than Ctenodus in retaining 
the Dipterine arrangement of radiating ridges on the tooth-plates ; in fact, 
Sagenodus and Ctenodus seem to afford an illustration of the fact, of not 
uncommon occurrence and great theoretical interest, that if one of two allied 
closely-related forms is more specialized in a certain region than tbe other, 
it will be less advanced in the structure of some other region. 
The comparison between the structures of Sagenodus and Ceratodus 
included in the description of tbe former genus seems to us to establish tbe 
descent of tbe latter from the former animal. 
The Dipnoi from tbe Old Red Sandstone probably lived in unusual 
conditions. J. Barrell has brought forward a mass of evidence to show that, 
in common with the other fish of the Old Red Sandstone, they were 
inhabitants of an arid region with seasonal rainfall, living in rivers which 
were liable to drj' up during part of the year and in shallow and 
impersisterit lakes. Although we believe that this view cannot be upheld 
in its entirety — for it is difficult to conceive of the Caithness flags being- 
deposited anywhere except in a very extensive and permanent sheet of 
water — it is undoubtedly well founded in its general conclusions. 
Tbe Coal-Measure Dipnoi lived under entirely different conditions in pools, 
often, as in the case of that in which the reof of the Low Main Seam at 
Newsham was deposited, of very considerable size and permanence. These 
pools seem to have lain in the midst of the coal-producing forests in a 
climate which was in no way arid. Thus this difference in habitat at once 
affords an explanation of the absence of any direct descendant of tbe 
Phaneropleuron line and the occurrence of a stock not known from the 
Upper Devonian. 
There remain for consideration the remarkable Uronemus and Concliopoma. 
These animals have been associated with one another, though only very 
doubtfully, by Traquair and Smith Woodward, because of the replacement 
in them of typical dental plates by isolated small denticles. 
Comparison of tbe figures and description given in this paper will show 
that there are no valid reasons for believing in tbe close affinity — that, in 
fact, they differ so greatly as to be in all probability merely functionally 
parallel modifications of very different stocks. Uronemus has a skull-roof 
retaining interfrontal and internasal bones, but much reduced by the loss of 
the bones on the lateral edge of tbe temporal region behind the squamosal, 
and of others forming the roof of tbe skull. Concliopoma, with a much more 
reduced skull-roof, loses the internasal, has a much enlarged interfrontal, 
exhibits a fusion of the frontal, intertemporal, and post-frontal, and retains 
