74 STRUCTURES OF SHARKS 



Structural Characters 



The definition of a shark emphasizes its cartilaginous 

 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, NC, 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 haemal 

 arch and spine, HER, 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, NP, 1C, 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, thpr 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 

 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 pre-spiracular 

 ligament. 



