VIII SKULL 223 
(Heterodontus) exhibits another and quite different modification. 
In this Fish the dorsal border of each palato-quadrate cartilage 
fits into a deep groove along the infero-lateral surface of the 
cranium, and is firmly attached thereto by ligament. Thus the 
first step is taken towards that more complete fusion of the two 
structures which is so characteristic a feature in the more typically 
autostylic Fishes like the Holocephali and the Dipnoi. Auto- 
stylism, whether incipient, as in Heterodontus, or complete, is to be 
regarded as a secondary modification, which may be independently 
acquired in widely different groups of Fishes, and is usually 
associated with the need of a firm and rigid support for an ex- 
ceptionally massive dentition.’ 
LE Fic. 129.—Lateral view 
of skull of Chimaera 
monstrosa. a.8.C, Posi- 
tion of anterior semi- 
circular canal; c.hy, 
cerato-hyal;  e.hy, 
epi-hyal ; fr.c/, frontal 
clasper ; h.s.c, posi- 
tion of horizontal 
semicircular canal ; 
1.0.8, inter - orbital 
septum ; 206.7, 6.2, 
lb.3, labial cartilages ; 
Mck.C, mandible ; 
Nv.2, optic foramen ; 
Nv.10, vagus  fora- 
men ; o/f.cp, olfactory 
capsule ; op.r, oper- 
cular rays ; pal.qu, 
palato - quadrate ; 
ph.hy, pharyngo-hyal, 
or hyomandibular ; 
p.s.¢€, position of posterior semicircular canal ; gw, quadrate region ; 7, rostrum. (From 
Parker and Haswell, after Hubrecht. ) 
In the Holocephali (eg. Chimaera”) the cranium retains its 
primitively cartilaginous condition, and assumes a somewhat 
peculiar appearance owing to the lateral compression and vertical 
growth of its inter-orbital and nasal regions (Fig. 129). There 
is a complicated series of labial cartilages in relation with the 
ventrally-placed nostrils and the upper and lower jaws. In the 
males of Chimaera and Callorhynchus, but not in Harriotta, a 
movable cartilage is attached to the cranial roof, and supports 
the frontal clasper. The skull is typically autostylic. Along 
1 Dollo, Bull. Soc. Belge Géol. etc. ix. 1895, p. 110. 
* Hubrecht, Niederlind. Archiv f. Zool. iii. 1877, p. 255. 
