522 



FISHES 



of limbs, limb-girdles, jaws, or branchial 

 arches, nor any evidence of the existence 

 of scales, denticles, or other exoskeletal 

 structures. 



Palaeospondylus has been regarded 

 as a Cyclostome, a view which derives 

 its principal support from the resem- 

 blance of the cirri -encircled orifice at 

 the anterior end of the skull to an 

 unpaired nasal or naso-pituitary aper- 

 ture, and perhaps some measure of 

 credence from such purely negative 

 evidence as the apparent want of limbs, 

 and of any structures comparable to 

 jaws. But even if it be admitted that 

 there is some reason for this view, it 

 is certain that Palaeospondylus obtained 

 a far higher grade of specialisation in 

 certain respects than any of the exist- 

 ing Cyclostomata ; the presence of calci- 

 fied vertebral centra and neural arches 

 is conclusive on this point. 1 Palaeo- 

 spondylus has also been compared with 

 a larval Arthrodiran and with a larval 

 Dipnoid. 2 



II. Ostracodermi. 



The Palaeozoic fish-like forms, which, 

 more as a matter of convenience than 

 as an expression of real kinship, it 



FIG. 312. Restoration of Palaf.o- i i -, . ,, 



spondyius. The figure shows has been customary to include in this 

 the ventral surface of the skull group, are amongst the earliest Craniates 



and a lateral view of the ver- /, , . , , 



Calcified o which we have any precise knowledge. 



tebral column, 

 cirri ; p.a, auditory capsule ; 

 t.p, nasal capsule (?) ; x, post- 

 occipital plate. (From Parker 

 and Haswell, after Traquair.) 



1 In a recently published and important con- 

 tribution to our knowledge of Palaeospondylus, by 

 Professor and Miss Sollas (Phil. Trans. 196, 1903, 

 p. 343), they describe structures on the ventral 



surface of the head, which they maintain to be branchial arches, as well as others 

 which, in their view, may represent hyomandibular and mandibular elements. 

 2 Graham Kerr, Proc. Camb. Phil. Soc. x. 1900, p. 298. 



