xvii ELASMOBRANCHI1 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 include supradorsals 

 and regularly alternating basi- and inter -dor sals. 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 inter branchial 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 limits 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 



