166 PROF. D. M, S. WATSON AND ME. E. L. GILL ON THE 
llie Roof of the Skull. 
It has been customary to dismiss the cranial roof o£ Sagenodus and 
Ctenodus as being composed of numerous small bones of rather indeterminate 
arrangement and doubtful homology, though bearing a considerable resem- 
blance to those of the roof of the skull in Dipterus. Incomplete skull-roofs 
of both genera were described, but not figured, by Hancock and Atthey 
(1872, p. 401), who clearly stated some of the points distinguishing skulls of 
Ctenodus tubercidatus (i. e. the genus Ctenodus as now understood) from those 
of C. obliguus (i. e. Sagenodus). Rough figures of the posterior roof-bones of 
the Sagenodus skull were given by T. P. Barkas (1873, figs. 244-246) and 
Miall (1881, fig. 1). Miall's figure was good as far as it went, except in 
the region of the bones that we call the intertemporals, where it is probable 
that he was misled by false suggestions of sutures. In a large number of 
examples, at any rate, we have met with no similar case, though monstrosities 
do occasionally occur ((;/'. fig. 6, A and 0, p. 170).* 
Of Barkas's three figures, 245 evidently represents the same specimen as 
was used by Miall for the right side of his figure, while fig. 246 represents a 
fragment seen from the underside and showing the shape regularly assumed 
underneath by the suture at the front of the "parietal." His fig. 244, that 
of the hinder -two-thirds of a skull-roof, is correct on the whole, but the 
posterior corners are missing ; the original is now in the British Museum 
(No. 45852). 
The length of an average skull-roof is about 5^ inches (14 cm.), but some 
were rather larger. The figure here given (fig. 1, p. 164), together with the 
variations represented in fig. 2, renders unnecessary a detailed description of 
the form of the bones. The bones are named in accordance with the sug- 
gestions of Watson and Day (1916, p. 42). Their degree of variability may 
be estimated by comparing the outline of any one bone through the series of 
figures, each drawn from an actual specimen, given in fig. 2, p. 165. It will 
be seen that though there is wide variation in details of shape, there is never 
any difficulty (apart from rare monstrosities) in recognizing single bones 
detached from the skull. 
In the state in which they occur in the shales the bones of the skull-roof 
are about 4 mm. thick in full-sized heads. Neighbouring bones are locked 
together by means of thin laminte, radially ribbed, springing from almost the 
lowest level of the thickness of the bones and fitting into the adjacent bone 
at the same level. Mr. H. Fletcher, late of the Zoological Museum, Uni- 
versit}' College, Reading, tells us that the bones in the skull-roof of Ceratodus 
forsteri are articulated in the same way. On the inner surface of the roof 
some bones encroach upon others further than they do outside, so that though 
the pattern on the two surfaces is nearly the same it is never identical. 
* In fig. 6, A, the right frontal (fr.r.) is quite abnormal in shape, and in fig. 6, C, the left 
intertemporal has entirely vanished, the region in which it should be being occupied by an 
■enlargement of the bones which normally surround it. 
