The principal part of the head sk2leton is a massive cartilaginous
box (chondrocranium), enclosing the brain, and including as a
result of growth and fusion the nasal and auditory capsules. The
eye capsules are free, and their accommodation at
the side of the cranium establishes the orbital
depression to be seen on the skull of all vertebrates.
This type of structure is obviously the basis of the
mammalian skull, except that in the latter the
cartilage mass is at once more restricted to its basal
portion, and is more specialized by its replacement
by definite bone centres.
It is, however, in respect of the visceral arches
suspended from the chondrocranium that the structure of the shark
skull is most illuminating. The first or mandibular arch forms
the upper and lower jaw. Its composition reflects that of the
external part of the auditory chain in the mammalian embryo, and
establishes a fact, for a long time scarcely believed by zoologists,
that in the history of mammals this arch has undergone a profound
change of function. The second or hyoid arch, though developed
to an extent out of keeping with the degree of development of the
primitive tongue, is nevertheless obviously homologous with the





Fic. 31. Shagreen
denticles of the
smooth _ dogfish,
Mustelus; enlarged.
hyoid arch of a mammal and in most sharks has an important
accessory function in the support of the lower jaw. Following
the hyoid arch are five ordinary or branchial arches supporting
the filaments of the gills and serving as pillars of the gill apertures.
These arches are then the parts of an extensive system out of which
those portions of the laryngeal cartilage which are of visceral rather
than tracheal origin have been specialized in mammals.
