190 PROF. D. M. S. WATSON AND MR. B. L. GILL ON THE 
part of the skeleton, — but that the general plan is the same. In the posterior 
region the correspondence is very close. The main points of difference may be 
broadly stated thus : in Ctenodus the two median bones {par. and i.fr.') are 
much smaller and the bones of the nasiil region much larger, uhile there are 
two pairs of bones in the frontal region instead of only one. As far as its 
skull-roof is concerned, Ctenodus, being nearer to the Devonian Dipterus, is 
presumably the more primitive ; and we may usefully think of the skull-roof 
of Sagenodus as being derived from that of Ctenodus by a gient reduction of 
the snout region together with an increase in size and eventual meeting of 
the two median bones, both processes together resulting in the compression, 
and finally in the fusion, of the frontal and post-Crontal on each side. 
As long ago as 1872 Hancock and Atthey correctly pointed out, as one of 
the main features distinguishing the skull of Ctenodus proper from that of 
Saqenodus (" Ctenodus obliquus "), that the two median bones of the roof are 
separated and that the hinder one consequently has a pointed instead of a 
concave anterior margin. Dr. Smith Woodward (1891, p. 250) refers to the 
same distinction when he states that there are " two median occipital plates '■" 
in Sagenodus and only one in Ctenodus. Ho also gives (pp. 252-3) the only 
extended description hitherto published of the skull-roof in Ctenodus, but 
the specimen on which he founded it (B.M., P. 5031), figured in pi. 4. vol. ii., 
of his catalogue, is not well preserved ; it shows for the most part impres- 
sions of the under surface of the bones, and their outlines are too indistinct 
for accurate representation. The same specimen was roughly figured by 
Fritsch (1889, text-fig. 156), but as he failed even to identify the middle line, 
his observations on it (p. 98) were not hel[)ful. 
Another specimen (now in the British Museum, P. 7300) was also roughly 
figured by Fritsch in his text-figure 155, and is represented in outline in 
our fig. 23, A, p. 192. It is chiefly remarkable as showing a considerable 
ossification in advance of the nasals, in the form of a radially-ribbed fan. 
Fragments of a similar fan in the Atthey Collection are shown in fig. 22j 
p. 191. Except at their outer edge, the bone composing them is as thick 
as that of any other part of the cranial roof. 
The presence of an internasal is a further point of distinction from 
Sagenodus. The form of intermisal shown in fig. 21, p. 189, of which two 
specimens are known, is possibly characteristic of Ctenodus cristatus. The 
form figured in B, fig. 23, p. 192, may similarly be characteristic of 
" C. inter imptus." 
The bones of the roof proper in Ctenodus seem to be about as variable in 
shape as in Sagenodus. The specimens we have examined show that the 
most viiriable elements are the interfrontal, internasal, and post-frontal 
(compare fig. 21, p. 189, with the outlines in fig. 23, p. 192). From the 
condition of the available specimens it would aj)pear ihat the elements of the 
cranial roof of Ctenodus were more firmly united than those of Sagenodus, 
