586 



PHYLUM CHORDATA : CLASS CYCLOSTOMATA 



(i) " The skull is apparently formed of calcified cartilage, and devoid 

 of discrete ossifications." An anterior part is comparable to the 

 trabecular and palatal region of a lamprey's skull ; a posterior part is 

 comparable to the parachordal region and auditory capsules. 



(2) " There is a median opening or ring, surrounded with cirri, and 

 presumably nasal, in the front of the head " (Fig. 329, n.). 



(3) " There are neither jaws nor limbs." 



(4) " The rays which support the caudal fin expansion, apparently 

 springing from the neural and haemal arches, are dichotomised (at least 

 the neural ones), as are the corresponding rods in the lamprey." 



Just behind the head lie two small oblong plates (Fig. 329, x.), 

 closely apposed to the commencement of the vertebral column, one on 

 each side. The notochordal sheath is calcified in the form of ring- 

 shaped or hollow vertebral centra with neural arches. Towards the 



Fig. 330. — Pterichthys milleri. Lateral view. — Restored by Traquair. 



tail the arches are produced into slender neiural spines, opposite which 

 are shorter haemal ones. 



It should be noted, however, that SoUas (1903) has argued that 

 Palasospondylus represents an early Amphibian. Graham Kerr (1919) 

 holds it to be a Dipnoan, "either larval or an adult form of small size 

 and primitive structure." 



Class Hypostomata or Ostracodermi 



Extinct forms without jawS; without a segmented axial skeleton in 

 the trunk, without any trace of girdles, with complex dermal armature, 

 with a head shield ; Silurian and Devonian, e.g. Pteraspis and Cepha- 

 laspis, both without paired limbs ; and Pterichthys and Bothriolepis, 

 with strange armoured paddles (probably not limbs in the ordinary 

 sense) fixed to the antero-lateral angles of the body-shield. Their 

 systematic position is still doubtful. They are the oldest known 

 Vertebrates. 



