552 PROP. W. B. BENHAM AND MR. W. J. DUNBAR ON [May 15, 



a process forwards to meet the great opistliotic (PI. XXXIX. 



^^- ^)- . . . 



The Supraoccipital is feebly ossified. Lying in the posterior 



dorsal groove and forming its floor, it is separated from the 



exoccipitals, however, by the epiotics — a very unusual condition 



(PI. XXXIX. fig. 4). Anteriorly it reaches the meeting-place of 



the two dorsal grooves. 



Otic Bones (PI. XXXIX. figs. 4, 5, 6, 8). 



Above the exoccipital lies the Epiotic, which is partly covered 

 by the post-temporal. The position of the post-temporal is shown 

 in PI. XXXVIII. fig. 1 and PI. XXXIX. fig. 8, but in the other 

 figures it has been removed. The two epiotics nearly meet one 

 another in the middle dorsal line, thereby excluding the occipital 

 from the sujjraoccipital, which, as Prof. Parker pointed out, is a 

 most unusual condition. The epiotic is a squarish bone as seen 

 from above, raised into a prominent ridge along the dorsal 

 surface, which forms the margin of the dorsal groove. 



The Pterotic is a large well-ossified bone extending from the 

 exoccipital to the posterior margin of the orbit. Dorsally it is 

 bounded successively by the frontal, the parietal, and the epiotic 

 bones, and ventrally it touches the sphenotic and prootic. The 

 outer edge forms a prominent ridge under which the hyoman- 

 dibular articulates. The pterotic is much grooved on its outer 

 surface, the ridges being very delicate while the grooves are deep. 



The Sphenotic is a more or less vertical bar at the posterior 

 region of the oi-bit, forming the post-orbital process. The lower 

 end slopes inwards and forwards to meet the upwardly-projecti]ig 

 process of the parasphenoid, and thus to form tlae post- orbital bar. 

 Above, it touches the under surface of the pterotic and anteriorly 

 the alisphenoid, while by a forward process it just reaches the 

 frontal. 



The Prootic lies below the ridge formed by the pterotic. It is 

 bounded anteriorly by the sphenotic, ventrally by the opistliotic, 

 and posteriorly by the exoccipital. 



The Opistliotic, as seen in side view, is a large bone forming a 

 plate above the parasphenoid and meets its fellow in the middle 

 ventral line. Relatively it is a larger bone than that of P. glesne, 

 and instead of the suture between it and the basioccipital being 

 vertical, the opisthotic passes backward over the forward plate 

 of the basioccipital to meet the exoccipital on the external surface 

 of the skull (PI. XXXIX. fig. 6). This external plate corresponds 

 to Parker's " oph." — the descending process. The other three 

 processes which exist on the inner surface of the cranial wall 

 ai'e not figured. If the side view of the skull of Pegalecus be 

 compared with that of the Salmon, it will be seen that the 

 opisthotic occupies the place of the prootic of the Salmon in its 

 relation to the exoccipital and basioccipital ; but Parkei''s dissection 

 shows the true interpretation of t-lie bone. 



