74 STRUCTURES OF SHARKS 
Structural Characters 
The definition of a shark emvphnatiee its cartilagitibn 
skeleton, investiture of shagreen, uneven (heterocercal) tail, 
and its separate and slit-like gill openings. Its more defi- 
nite characters may well be summarized in the accompany- 
ing figure (Fig. 83). 
I. The sKELETON is cartilaginous (cf. Fig. 83, 84, and 
p. 252), sometimes calcified generally, but always (in recent 
forms) lacking in dermal bones. Behind the simple, trough- 
like brain case the vertebral rod, beginning at the occip- 
ital condyles, is clearly segmented ; the notochord is often 
retained, especially in the tail region, VC, but is encroached 
upon by the cartilaginous rings, centra, C, arising metamer- 
ally in its sheath (Fig 85). The vertical supports of each 
centrum include a well-marked ventral plate, the hemal 
arch and spine, HBR,—which in the tail region probably 
represents as well the cartilaginous elements of the fin 
support,—and a pair of small dorsal plates, the neurals 
and interneurals, VP, /C, each capped by a neural spine, — 
NS. The fin supports compare closely in structure with 
the vertebral processes; they form a large part of the 
functional fin, and preserve clearly, both in basal and 
radial parts, their metameral character. This segmental 
arrangement is also characteristic of the supporting ele- 
ments of the cavity of the mouth and throat. These con- — 
stitute the “visceral arches” (cf. p. 256) which pass — 4 
backward from the rim of the mouth to the region of the — 
pectoral fin. The first visceral arch strengthens the rim of 
the mouth; it is margined with teeth and functions as jaws,* 
* The writer believes that the upper element of the mandibular arch is to 
be regarded as the palatoquadrate cartilage, rather than the ee 
ligament. H 
