298 Mr Graham Kerr, The Zoological Position 



The Zoological Position of Palaeospondylus, Traquair. By 

 J. Graham Kerr. 



[Read 19 February 1900.] 



In the course of my work on the development of Lepidosiren, 

 having occasion to examine some skeletons of larvae which had 

 died and become macerated, and from which the heavy lower jaw 

 had dropped off, I was struck by certain resemblances in their 

 structure to that described by Traquair for Palaeospondylus 1 . 

 Although these resemblances may possibly prove to be of a 

 superficial character I think that in the meantime, looking to 

 the very great importance of such a form as Palaeospondylus, 

 they are worth while calling attention to by a short note. 



One of the most striking features about Palaeospondylus 

 appears to be the pair of curious post-occipital plates or rods, 

 projecting back from the hind end of the skull on either side 

 of the vertebral column. These, though so conspicuous and 

 striking, have so far received no morphological explanation. It 

 is interesting to note then that in the young Dipnoan we have 

 a pair of very similar structures in the so-called cranial ribs. It 

 is suggested that the two structures are actually homologous, 

 and in that case it would follow that Palaeospondylus is not a 

 Cyclostome but a young Dipnoan. It accordingly becomes of 

 interest to see whether the other features of Palaeospondylus as 

 described by Traquair are or are not understandable on this view. 

 A comparison of Giinther's figure of the ventral surface of the 

 skull of Ceratodus 2 with Traquair's figure of his most perfect 

 skull of Palaeospondylus 3 is enough to show the remarkable 

 agreement in the general features of the two skulls in their 

 posterior portion. In each case we have a median ridge (in 

 Ceratodus covered in by the parasphenoid) flanked on either side 

 by a deep depression and tapering backwards towards the point 

 where it is continued into the vertebral column, and attached on 

 each side close to the junction of skull and backbone — the cranial 

 rib (labelled x, curiously enough, in each of the figures referred 

 to !). Further forward the resemblance is less striking but even 

 here it only requires the slight modification of the pterygo- 

 palatine teeth in the Ceratodus skull, their antero-posterior axes 

 being increased in length and becoming parallel to one another 

 and their lateral projections becoming more prominent (by no 



1 (1) On the fossil fishes found at Achanarras Quarry, Caithness. Ann. and 



Mag. Nat. Hist., Series 6, Vol. vi. p. 479. 



(2) A further description of Palaeospondylus Gunni, Traquair. Proc. Roy. 

 Phys. Soc. Edin. Vol. xn. p. 87. 



(3) A still further contribution to our knowledge of Palaeospondylus Gunni, 



Traquair. Proc. Roy. Phys. Soc. Edin. Vol. xu. p. 312. 



2 Phil. Trans. 1871, Pt. n. Plate 34, Fig. 3. 



3 (3) Plate ix. Fig. 5. 



