212 PROF. D. M. S. WATSON AND MR. K. L. GILL ON THE 
be clearly the case, the tooth-plate on the pre-articular of Dipterus be a 
development o£ this customary granulation, then the similar upper jaw 
structure on which it grinds must have been derived from the similar 
granulation on the pterygoid. It is improbable that this large tooth-bearing 
bone on the palate can be a conjoint pterygoid and palatine as is usually 
believed, because the palatine teeth of Osteolepids are always long tusks, 
unlikely in any case to be worked up iuto so typical a crushing structure as 
the Dipnoan dental plate, and were the palatine preserved, we should 
expect to find that the lower plate was of coronoid derivation. Nothing in 
the structure of the Dipterus lower jaw supports this view, which can, how- 
ever, never be disproved because of the possibility (although we think 
general great improbability) of a fusion of bones. 
The opercular apparatus of D. valenciennesi is readily interpretable in 
comparison with that of an Osteolepid. Its unusual feature, the direct 
contact between the principal gular and the sub-opercular, is probably due to 
the great reduction of the hinder lateral gulars which is associated with the 
shortening of the jaw. The meeting of the two rows of lateral gulars in 
the front is paralleled in Me<jalw.hlhys (No. 28308, Museum of Practical 
Geologj). 
The problem of the relationships of the known Dipnoan genera to one 
another is still incapable of satisfactoi-y solution, because of paucity of material. 
The position of Dipterus valenciennesi, at the base of the series, seems to be 
made certain not only by the fact that it is actually the oldest known species, 
but by the close comparison which can be drawn between its structure and 
that of an Osteolepid. 
From this form the series D. platyceplmlus, Fentlandia, Scaumenacia, 
F haneropleuron, first suggested by Prof. Dollo and supported from new 
evidence by Watson and Day, seem to have arisen. These forms first 
appear in time in the order named, and .show a steady progressive change, 
resulting in a reduction of the dermal bones in the front of the head, a 
reduction of the ossifications in the chondrocranium, a loss of the inter- 
frontal and internasal elements, and a gradual fusion of the median fins 
with one another. All known features of their-structure are consistent with 
direct descent of any form from that which precedes it. 
The new facts about the structure of Sagenodus and Ctenodus brought 
forward in this paper show that Watson and Day were not justified in 
separating them widely from one another and in deriving Sagenodus from 
P haneropleuron. 
The descriptions we have given show that the two animals are closely 
related, that indeed the Lower Carbonifei-ous " C. interruptus" is in its teeth 
in many respects intermediate between the C!oal- Measure forms of the two 
genera. Ctunodus has the less reduced skull-roof, which by its retention of 
