a fish is given in Figure 2, Sheet 16, and this should
be carefully compared with the corresponding small figure given of the
vascular system of our other types.

{Lines from Second Edition only.}
[The blood of the dog-fish resembles that of the frog.]

Section 9. The internal skeleton, as we have said, is entirely
cartilaginous, and only those parts which are pre-formed in cartilage in
the skeletons of the higher types are represented here. The spinal
column consists of two types of vertebrae, the trunk, bearing short,
distinct, horizontally-projecting ribs (r.), and the caudal. The
diagrams of Figure 5 [(Sheet 18)] are to illustrate the structure of the
centrum of a dog-fish vertebra; C is a side view, D a horizontal median
section, A and B are transverse sections at the points indicated by -B
and A- [A and B] respectively in Figure C. -(By an unfortunate slip of
the pen in the figure, A was substituted for B; section A corresponds
to line B, and vice versa.)- The vertebrae are hollowed out both
anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord
runs through the entire length of the vertebral column, being
constricted at the centres of the centra, and dilated between them.
The neural arch above the centrum, and containing the spinal cord, is
made up of neural plates (n.p.), and interneural plates (i.n.p.),
completed above by a median neural spine (n.s.). In the caudal region,
instead of ribs projecting outwardly, there are haemal processes,
inclined downwards and meeting below, forming an arch, the haemal
arch, containing the caudal artery and vein-- the vein ventral to the
artery-- and resembling the neural arch, which contains the spinal
cord above, in shape and size.

Section 10. The pectoral limb and girdle (Figure 4, Sheet 16) have
only a very vague resemblance to the corresponding structures in the
rabbit. The girdle (g.) is a transverse bar lying ventral to the
pericardial wall, and sending up a portion (sc.), dorsal to the attachment
of the limb, which answers to the scapula and supra-scapula of the forms
above the fish. Three main cartilages, named respectively the
pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the
limb. With these, smaller cartilaginous plates, rods, and nodules
articulate, and form a flattened skeletal support for the fin.

Section 11. The pelvic girdle and limb (Figure 2, Sheet 15) are
similar in structure, but the pro-pterygium and mes