288 
PROFESSOR H. G. SEELEY ON THE STRUCTURE, ORGANIZATION, 
while at the anterior fracture the double vomera make a dilference from the vomer of 
Dlcynodon ; so that I regard the palate in the two groups as formed on different types. 
The Theory of the Anomodont Skidl. 
Ouly when it is established that the Dicynodontia, and therefore the allied Anomo- 
donts, differ as sub-orders from the Pareiasauria, does it become possible to realize 
the magnitude of the changes which a skull may undergo in the same natural group 
of animals, and also how considerable is the gap which remains to be filled in before 
the most Mammalian type of Anomodont could be transformed, in so far as its skull is 
concerned, into a Mammal. We are at present ignorant of the modes of elaboration 
of such change, beyond knowing that certain bones have to be obliterated from the 
Reptilian skull to make it Mammalian. But whether that loss was brought about by 
the peculiarly Reptilian elements dwindling in size until they disappeared, while the 
peculiarly Mammalian elements augmented their growth in a corresponding way to 
take their places, or whether the Mammalian type implies such an osteological retro¬ 
gression as the loss of some fundamental segmentations which divide bones from each 
other in ancestral types, cannot be determined, even with j^robability, without the aid 
of theory. I liave already regarded the skull as a more primitive part of the skeleton 
than the vertebral column, less specialized ; so that it preserves structures, which 
originally existed in the vertebral column, long after the vertebrse have lost them.'^ 
I have compared the roof bones of the skull to the roof bones of the vertebral 
column which exist in those plagiostomous Fishes in which theie is a superior 
intercalary segment introduced between two adjacent neural arches. This seems 
to me to explain the presence in the skulls of lower Vertebrates of those bones 
which have been termed inter-parietal, post-frontal, and pre-frontal. The inter¬ 
parietal persists in some Mammals, and in some orders is absent. When it is 
absent it is manifestly blended with the supra-occipital. I see no reason for 
thinking that the inter-parietal gradually disapjjeared, and that the supra-occipital 
grew at its exjiense ; but, just as the inter-centrum may become blended with the 
centrum, so these bones have blended with some associated elements in the skull. 
The argument in favour of this interpretation rests upon the fact that in those Fishes 
in which the intercalary neural elements are present the neural arches form an 
unbroken continuous tube, whereas in those animals in which they are not found there 
are more or less appreciable gaps between contiguous neural arches ; and, if any element 
in the cov^ering of the skull were absent, it seems more probable that a fontanelle 
would result than that the other bones would take its place. Similarly, in certain 
Chelonians, like Podocnemis and Rhinochelys, the pre-frontal bones retain the distinct 
individuality which characterizes them in other Reptilia ; whereas in the majority of 
Chelonians this individuality is lost, and the pre-frontal bones are not difierentiated 
* “ The History of the Skull,” King’s College Science Society, October, 1882. 
