STRUCTURE OF CERTAIN PAL^.OZOIC DIPNOI. 211 
Thus we are driven to conclude that the Dipnoi, iiltliough they have 
undoubtedly been derived from a stock whose general niorpliology was 
Osteolepid, were separated from that stock before its members acquired their 
most peculiar and diagnostic features. 
(Comparison of the head of Dipnoi with that of an Osteolepid, the homo- 
logies of whose bones are obvious in the light of the structure of Palseozoic 
amphibia, throws great light on the determination of the elements of the 
sknll. 
The ver}'- interesting observations recorded by Prof, (xoodrich in his 
general work on Fishes (1909) were the first correctly to lay down the main 
lines of the com[)arison between the skulls of Dipnoi and Osteolepids, 
especially in their recognition of the distribution of the lateral-line canals on 
the top of the head. 
Gregory (1915) further extended this comparison, and finally a comparison 
of the skull-roof of D. valencienned with that of the Osteolepids led 
Watson and Day to an identification of nearly all the bones which compose 
its apparently inexplicable mosaic. 
Tliis interpretation depends on the belief that in JJ. platycephalus the 
tabulars and post-parietal have fused with the parietals and supratemporals. 
The lack of a specimen of this species with Ihe skull in articulation with the 
trunk renders the evidence incomplete, but the fact that the posterior row of 
three bones in all later Dipnoi houses the occipital cross-commissure of the 
lateral line shows that it includes these bones ; and the well-known pair 
of grooves on the median occipital, which are for a line of pit organs and are 
obviously homologous with a simihir pair of grooves on the parietals of 
Osteolepids and some other fish, show that this bone includes the parietals. 
In the lower jaw of Dvpterus the dentary, angular and arlicular are readily 
identifiable, and the relations of the splenial and post-splenial to the dentary 
and angular are so exactly those which obtain in Osteolepids and indeed in 
Labyrinthodont amphibia as to leave no doubt of their homologies The 
only remaining bone, that which bears the tooth-plate, cannot be the splenial, 
as has been previously believed, because an undoubted splenial occurs in the 
same jaw. It can only be pre-articular or coronoid : the application of its 
hinder end to the inner surface of the articular, its large size and position on 
the inner surface of the ramus, show that it is a pre-articular, and the 
symphysis which it makes with its fellow can be matched in the case of 
the pre-articular of Megalichthys. 
In all Osteolepids and Labyrintbodonts the teeth-bearing bones of the upper 
and lower jaws have certain quite definite relations to one another. The 
dentary bites within the maxilla, the teeth on the coronoids interlock with 
those on the palatine and ectopterygoid, and the upper edge of the pre- 
articular, which is usually covered with a granulation of sm-.dl denticles, faces 
although it does not touch the similarly armed pterygoid. If, as seems to 
