ELASMOBRANOHS. 
1071 
projects downwards in an angle, known as the basilar 
angle". But in the more recent forms of Sharks and 
in the Rays these articulations have been loosened, and 
are replaced by ligaments, the postorbital process and 
the basilar plate having meanwhile suffered reduction/'. 
The Chi mamas have undergone an opposite development, 
the palatoquadrate cartilage (tig. 294, qpg) being con- 
fluent with the skull — whence the name of Holoce- 
phali — and forming a, firm floor to the orbit. Within 
the skull, above and just behind the basilar angle, lies 
the Turkish saddle (with the infundibulum , glandula 
pituitaria, and canalis caroticus ), and here the septum 
between the orbits is pierced by a transverse canal, in 
and around which the musculi recti of the eye have 
their insertions, and which is consequently homologous 
with the posterior orbito-muscular canal present in most 
of the Teleosts. In the orbital region the roof of the 
skull (the forehead) is sometimes (in the Rays) imper- 
fectly closed, being completed by a membrane (frontal 
fontanelle, fig. 300, fonf). The anterior limit of the 
orbits is formed by the preorbital process (figs. 298— 
300, prob), which is pierced, as well as the more or 
less arched and expanded roofs of the orbits (with their 
supraorbital foramina, fig. 298, fspo ), by ramifications 
of the ophthalmic branch of the trigeminal nerve. The 
canal (preorbital canal, figs. 294 and 298, cp) that 
traverses the upper part of the preorbital process for 
this purpose either opens on the top of the skull (fig. 
299, cpf) and soon descends again into the cartilage 
of the ethmoid region (ethmoidal canal, fig. 294, re), 
or proceeds uninterruptedly, without appearing on the 
surface, into the said cartilage, or leads to a deep in- 
cision on each side of the skull (fig. 299, ie), the limit 
between the orbital and ethmoid regions. In the lower 
part of the preorbital process runs another canal, the 
so-called orbitonasal canal (fig. 294, on), to receive an- 
other ramification of the said ophthalmic nerve. Behind 
the optic foramen there often projects within each orbit 
a cartilaginous rod to support the eyeball, this rod being 
the so-called eye-stalk. The tip of each preorbital pro- 
cess sends out in the Notidaniclce (the most primitive in 
type of the surviving Sharks) a backward process, 
which in the Rays (fig. 300, pob) becomes a free car- 
tilage jutting out laterally and jointed by a well-deve- 
loped articulation with the preorbital process and the 
nasal capsule. By Parker/ this cartilage was inter- 
preted as a vestige of a. preoral (situated before the 
mouth) visceral arch; but it would seem to have most 
in common with the preorbital bone of the Teleosts, 
especially as this appears in the Cobitidce. In the 
Rays it meets the prorsal basal cartilage (propte- 
rygium) of the pectoral fins, and is most highly de- 
veloped in the Electric Rays ( Torpedines ) and Eagle- 
Rays ( Myliobatidce ). 
The ethmoid region composes the anterior limit 
of the cranial cavity, being the product of the strong 
expansion and development of cartilage attained by the 
side-angles of the embryonic trabeculae (fig. 298, ctr ). 
Medially this mass coalesces into a septum, the inter- 
nasal cartilage, between the large nasal capsules (figs. 
294 and 298 — 300, N), each of which is pierced with 
a large foramen for the olfactory nerve. On the upper 
side of the skull there appears at this point, in most 
of the Elasmobranchs, the large prefrontal fontanelle 
(figs. 299 and 300, fonpf ), filled with a loose, mucoid 
connective tissue, the backward continuation of which 
passes, in the interior of the skull, into the hard mem- 
brane of the brain ( dura mater). The lateral parts of 
the ethmoid region are formed by the large nasal cap- 
sules (N), which are of an inverted bowl-shape, arched 
above, open below, and the posterior walls of which 
coalesce on each side with either preorbital process. 
The structure of these capsules originates, it is true, 
from the side-ends of the embryonic trabeculae; but 
also includes up to three so-called labial cartilages 
(figs. 294 and 298, ln i and Inf), originally free, which 
form the margin and marginal valves of the nostrils. 
The rostral region, which attains its greatest de- 
velopment in the Rays and Saw-fishes — being pro- 
duced in the latter into the so-called saw with its large 
lateral serrations and its two pairs of longitudinal ca- 
nals, the inner pair continuations of the above-men- 
tioned preorbital canals — is least developed in the 
most primordial Sharks, where it forms a compressed 
and narrow, or depressed and broad prolongation of 
the cranial floor. In most Sharks, however, the snout 
is furnished on each side with a supporting cartilage 
(figs. 298 and 299, Ir), originally a so-called labial 
a Cf. the Sturgeons, see above p. 1045, fig. 284, D. 
b An exception to this is the Hammerhead (Zygceiia), in which the postorbital process as well enters into the laterally elongated orbits. 
c Skull of Sharks, Skates, Trans. Zool. Soc. Lend., vol. X, p. 224 (ao). 
Scandinavian Fishes. 
135 
