THE NORTHERN RIBBON-FISH. 5 
The anterior face is covered above by the sphenotic; below, it is exposed and 
articulates with the prootic. The two faces meet in a rounded edge, which 
articulates anteriorly with the alisphenoid, and above with the frontal. The latter 
bone shuts it out from the internal surface of the brain-case above. A small portion, 
together with the intervening cartilage, serves to complete round the pillar formed 
by the epiotic and prootic the canal which seems to lodge the horizontal canal of 
the ear. 
The sphenotics are rounded elements which present a smooth outer surface, 
slightly hollowed in the middle, and a deeply pitted internal surface. From the 
outer surface a thin splinter descends and meets a corresponding upgrowth from 
the parasphenoid, thus forming the post-orbital pillar. The sphenotic articulates by 
a large surface with the pterotic, and the posterior edge of that surface articulates 
with the epiotic ; anteriorly it articulates with the alisphenoid, below with the 
prootic, above with the frontal. 
Prootics.—In the figures accompanying this paper we have represented this bone 
under the letters op., believing it to be the opisthotic. It is figured and described 
in Regalecus under the latter name by PARKER, who saw sufficient cause for his 
identification in the presence of a small separate prootic, imperfectly ossified, which 
is not present in Trachypterus. However that may be, and whether or no a true 
opisthotic be incorporated with it, the bone under consideration here can only be 
identified with the prootic on account of its large size, its union with its fellow in 
the middle line, its relation to the fifth and seventh nerves, and to the representa- ~ 
tive of the canal for the ocular muscles. It consists of a body from which diverge 
numerous processes, as follows:—(1) A process, stout and rounded, ascending to 
meet a corresponding process from the epiotic (Pl. II., Fig. 5); (2) a process, 
outwardly directed, which meets the sphenotic; (3) a flat process which articulates 
with the lower margin of the alisphenoids, it is directed forwards, and contains the 
foramen for the fifth and for the seventh nerves—the first division of the fifth nerve 
passes internal to the post-orbital pillar, the second and third divisions and the 
seventh nerve emerging posterior to it; (4) a thin descending lamina, which is 
continued round on to the posterior end of the bone, and articulates by cartilaginous 
intervention with the corresponding descending process of the basi-occipital, below 
with the parasphenoid, and above with the anterior process of the ex-occipital, it is 
also continued behind the epiotic process, and articulates with the pterotic; (5) a 
horizontal process which stands inwards and meets its fellow to form part of the 
floor of the brain-case. This last is bony above and cartilaginous below, and is 
attached behind to the basi-occipital. This process in each case articulates also 
anteriorly with one of the forks of the basi-sphenoid. 
Parietals.—The parietal is not seen in the complete skull until the post-temporal 
is removed; it forms the upper part of the floor of the pit for that bone. It 
articulates externally with the pterotic; posteriorly with the supra-occipital and 
