XVII ELASMOBRANCHII—-HOLOCEPHALI 467 
they seem to represent a divergent and specialised offshoot from 
some primitive Elasmobranch type, and while retaining most of 
the essentially distinctive features of their ancestors, they have 
acquired, perhaps independently, certain characters distinctive 
of the Teleostomi, combined with others peculiar to themselves. 
In the few surviving genera agreement with the Elasmobranchs 
is to be seen in the wholly cartilaginous condition of the endo- 
skeleton and the complete absence of cartilage- and membrane- 
bones. The vertebral column is acentrous and ribless, and the 
notochord is persistent ; the dorsal arcualia imclude supradorsals 
and regularly alternating basi- and inter-dorsals. The limbs 
and limb-girdles are essentially Elasmobranch. Dermal denticles 
are present, either locally, or, as in some of the fossil types, in 
the form of a general investment. The brain and the reproduc- 
tive organs agree more closely with the corresponding structures 
in the Elasmobranchs than with those of any other Fishes, and 
the agreement extends to the large size of the eggs and their 
enclosure in horny egg-cases. In both groups the nostrils are 
connected with the mouth by oro-nasal grooves; the hyoidean 
hemibranch is a true gill, and there is no air-bladder. The 
Holocephali also agree with the Elasmobranchs in retaining such 
primitive features as an intestinal spiral valve and a conus 
arteriosus. On the other hand, indications of specialisation in 
the Teleostome direction are to be noticed in the tendency to 
the concentration of the branchial arches towards and beneath 
the skull; the reduction of the interbranchial septa to the 
extent that they are no longer continuous with the skin, and 
the gill-filaments project beyond their outer margins; the 
presence of an operculum; the suppression of the spiracles ; 
and the absence of a cloaca, the rectum opening externally by 
an anus in front of the urino-genital apertures. Among the 
more notable features evolved within the lmits of the group 
mention may be made of the autostylic condition of the skull, 
probably an adaptive modification induced by the large size of 
the crushing dental plates which have taken the place of 
ordinary teeth; and the singular development of anterior and 
frontal “ claspers.” 
The group is one of great antiquity. Apart from the isolated 
spines or “ichthyodorulites” common in Devonian and Carboni- 
ferous strata, some of which are probably the frontal or the 
