184 FISHES CHAP. 
pointed, enamel-tipped spine of dentine which protrudes through 
the epidermis, and projects backwards on the surface of the body, 
but is firmly fixed in the skin by the basal plate with which it 
is continuous. The centre of the under surface of the basal plate 
is perforated for the entrance of the blood-vessels which pass to 
the cellular pulp in the axis of the spine. In the adult Fish 
the denticles form a fairly close-set covering to the whole body, 
including the head and even the surfaces of the fins, and are 
larger on the dorsal than on the ventral surface (Fig. 99). In 
the Rays (Raia) they are more sparsely scattered, and in different 
parts of the body may form spines of considerable size for offen- 
Fic. 99.—Surface view 
of the dermal den- 
ticles of Scyllium 
sp., showing their 
arrangement in 
oblique __ transverse 
rows. 5, Basal plate ; 
c, canal which per- 
forates the basal 
plate and becomes 
the axial pulp-cavity 
of the spine; /0, 
intersecting fibrous 
bands of the dermis ; 
Ss, spine; in the 
spine of one scale the 
dentinal tubules are 
shown. The smaller 
denticles are those 
most recently 
formed, (After 
Klaatsch.) 
sive or defensive purposes. The spines vary greatly in shape in 
different members of the group, sometimes being acutely pointed, 
and sometimes flattened or depressed, and often they are furnished 
with smaller accessory spines developed at their bases or from 
the surface of the basal plate. An arrangement of the denticles 
in oblique transverse rows is observable in some genera (e.g. 
Scyllium). In the Saw-Fishes (e.g. Pristis) the denticles 
which fringe the lateral margins of the long flattened rostrum 
are not only enormously enlarged, but are implanted in sockets 
and form the teeth of the saw (Fig. 262). In the Holocephali 
the smooth skin is almost entirely devoid of exoskeletal struc- 
tures, but dermal denticles are present on the frontal and anterior 
claspers, and in the young there may be a double row of small 
denticles along the back. 
