184 PROF. D. M. S. WATSON AND MR. E. L. GILL ON THE 
operculum. In the restoration (fig. 20, p. 186) we have assumed that the 
edge of the operculum overlapped the thin flange of the sub-operculuin. 
A bone labelled " subopercular ? " was figured by Williston (1899, pi. 36, 
fig. 3). Though it is not much like our bone in shape, it may possibly be 
another form of it. 
The SJioidder- Girdle. 
The first bone of the shoulder-girdle to be recognized was the clavicle, 
which was correctly described by Hancock and Atthey (1872). Since then 
the only investigator who has contributed usefully to what is known of this 
part of the skeleton is Miall, who not only published figures (rough and 
imperfect it is true) of the clavicle (1881, fig. 11, " coraooid "), but also 
figured the cleithrum (fig. 10, " scapula "), till then unknown, and made 
(pp. 296-7) some suggestions, now in the main confirmed, as to the way 
in which these bones were arranged in the girdle. Miall added another 
bone (fig. 9) as the " supra-scapula," but we can find nothing like it, and 
it certainly forms no part of the shoulder-girdle of Sagenodus. Fritsch 
discussed the shoulder-girdle at length, but the upshot, in the restoration 
which he gave in text-^gui'es 158 and 160, p. 81j was anything but a happy 
application of his perfectly sound principle that Ceratodus was the best 
guide to the structure of " Ctenodus." 
In the case of the shoulder.girdle, again, the figures here given (figs. 17, 
p. 182, and 19, p. 185) may largely take the place of a detailed description ; 
and here again, as with the lower jaw, there is such a strong similarity to 
the corresponding parts in Ceratodus that the comparative outlines given 
in fig. 19 are practically self-explanatory. 
The history of the discovery of the clavicle and cleithrum has been 
indicated above. The third bone, the post-temporal, has also long been 
known. It was fully discussed by both Miall and Fritsch, and on the 
strength of a certain resemblance to the combined squamosal and quadrate 
of Ceratodus they both figured it as the squamosal (Miall, fig. 6 ; Fritsch, 
text-fig. 153, p. 75). The clue to its real nature, as well as to the arrange- 
ment of the shoulder-girdle in general, was given by a fine specimen in the 
Atthey Collection, the pertinent portion of which is represented in fig. 18, 
p. 183. On its upper surface this specimen is chiefly noteworthy as being 
the only example we have met with which shows the operculum in its natural 
relation with the skull. It also shows the upper end of the right cleithrum 
from its outer aspect. On the under surface, as it was left by Atthej', the 
most prominent objects were the two clavicles, lying in nearly their natural 
position ; the removal of masses of overlying scales has disclosed other 
structures which appear in our figure (fig. 18, B, p. 183). The right 
cleithrum and clavicle are in natural articulation. Underlying the left 
clavicle are the two post-temporals, the left one showing its strong anterior 
process projecting beyond the clavicle in front. Underlying the post- 
