o-pterygium are
absent, and the cartilage answering to the meta-pterygium goes by
the name of the basi-pterygium. In the male, but not in the female, the
pelvic fins are united behind the cloaca, and there are two stiff grooved
copulatory organs, the claspers (cl. in Figure 1), which have a
cartilaginous support (cl.c.). These claspers form the readiest means
of determining the sex of a specimen before dissection.

Section 12. The skull consists of a cartilaginous cranium, and of jaw
and visceral arches. The cranium persists throughout life, in what
closely resembles a transitory embryonic condition of the higher
types. There is a nasal capsule (na.c.), a brain case proper, and
lateral otic (auditory) capsules (ot.c.) containing the internal ear.
(This should be compared with the frog's embryonic skull.) The upper jaw
has a great bar of cartilage, the palato-pterygoid, as its sole support;
the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane
bones-- is, of course, not represented. In the frog this bar of cartilage
is joined directly to the otic capsule by a quadrate portion, but this is
only doubtfully represented in the dog-fish by a nodule of cartilage in
the pre-spiracular ligament (p.s.). The lower jaw is supported, by
Meckel's cartilage (M.C.). The hyoid arch consists of two main
masses of cartilage, the hyomandibular (h.m.), and the ceratohyal
(c.h.); the former of these is tilted slightly forward, so that the gill
slit between it and the jaw arch is obliterated below, and the cartilage
comes to serve as the intermediary in the suspension of the jaw from
the otic mass. There are five branchia[l] arches, made up pharyngo-,
epi- and cerato-branchials, and the ventral elements fuse in the
middle line to form a common plate of cartilage. Outside these arches
are certain small cartilages, the extra branchials (ex.b.) which,
together with certain small labials by the nostrils and at the sides of
the gape, probably represent structures of considerably greater
importance in that still more primitive fish, the lamprey. The deep
groove figured lateral to the otic capsule is the connecting line of the
orbital and anterior cardinal sinuses; the outline of the anterior
cardinal sinus in this figure and in Figure 1 is roughly indicated by a
dotted line.

Section 13. Figure 3a is a rough diagram of the internal ear-- the only
auditory structure of our type (compare Rabbit, Sheet 7). To dissect
out the audit