ox TBtE ANATOMY OF FISHES. 
83 
horizontally forwards over the dorso-lateral edge of the centrum of the first vertebra 
and beneath the lower extremity of the claustruin, and finally terminates on the 
lateral margin of the dorsal surface of the basioccipital in a characteristic spoon- 
shaped expansion, which is concave on its inner surface and convex externally (figs. 4 
and 5, sc.s.). The spatulate process is so adapted, as regards shape and size, as to 
accurately fit into and completely occupy the semicircular notch in the posterior 
margin of the exoccipital, and consequently to form by its inner concave surface the 
outer wall of the atrial cavity of its side, and completely close the corresponding 
external atrial aperture (figs. 4, 5, and 9, sc.s.). The margin of the concavo-convex 
portion of the spatulate process is attached by fibrous tissue to the lips of the atrial 
aperture, but the connection is nevertheless sufiiciently loose to admit of the process 
undergoing a slight inward or outward movement, according to the direction in which 
the scaphium rotates on its condylar process, and any such movement will necessarily 
lead to a slight diminution or increase in the size of the atrial cavity. Enlargement 
of the atrial cavities will take the form of lateral extension round the concave 
posterior edge of the root of the exoccipital {vide fig. 8, at.a.). On the inferior side 
of the junction of the spatulate and ascending processes there is a small spherical 
cartilage-tipped condyle, which fits into a cup-shaped socket provided for it on the 
dorso-lateral surface of the centrum of the first vertebra (figs. 10 and 11, sc.c. ; also 
fig. 5, sc.). The inner extremity of a stout tendinous interossicular ligament is 
attached to the outer surface of the spatulate process, and serves to connect the 
scaphium with the anterior extremity of the tripus (fig. 12, i.lg., also figs. 9 and 4). 
Like the scaphium the intercalarium consists of ascending and horizontal processes 
united at nearly a right angle (figs. 9 to 12, in., also figs. 4 and 5). Both processes 
are extremely slender and brittle. The ascending process (fig. 10, in.a.) lies in the 
fibrous wall of the neural canal behind and parallel to the corresponding process of the 
scaphium and immediately in front of the arch of the third vertebra (figs. 4 and 5, in.). 
At its lower extremity the process becomes continuous by a slightly open curvature 
with the horizontal portion of the ossicle (figs. 10 and 12, in.h.). The latter process 
is directed outwards and a little forwards, and eventually terminates in a discoidal 
expansion imbedded in the substance of the interossicular ligament exactly midway 
between the insertion of the latter into the spatulate process of the scaphium and its 
external attachment to the anterior end of the tripus (fig. 12, in.h.). The inter- 
calarium differs from both the trlpus and scaphium in having no connection or articula- 
tion with any vertebral centrum or other part of the skeleton ; nevertheless, its 
ascending process is capable of a limited amount of rotation on its own axis as the 
horizontal process moves backwards or forwards with the lateral motion of the tripus 
and the consequent pull of the interossicular ligament. 
The interossicular ligament (figs. 9, 12, 21, and 22, i.lg. ; also fig. 4) is a short but 
relatively to its length, extremely thick, transversely-disposed ligament, composed of 
glistening tendinous fibres of a yellowish colour, and stretching between its attach- 
M 2 
