CHAPTER XX 



APPENDIX TO THE FISHES : PALAEOSPONDYLIDAE OSTRACODERMI 



HETEROSTRACI OSTEOSTRACI ANASPIDA ANTIARCHI 



ARTHRODIRA. 



Ix this chapter it is proposed to treat of certain fossil " Fishes " 

 which, from our ignorance of much that is essential to a 

 proper estimate of their true relationships, cannot at present 

 be referred to any of the recognised primary groups of Fishes. 



I. Palaeospondylidae. 



The interesting little fossil, Palaeospondylus gunni, 1 discovered 

 in the Lower Old Red Sandstone of Caithness, and first described 

 by Traquair, represents the calcified endoskeleton of an elongated 

 fish-like organism about an inch, or not exceeding two inches, 

 in length. The vertebral column consists of a series of broad, 

 calcified ring-like centra, destitute of ribs, but possessing neural 

 arches and spines, and in the caudal region haemal arches and 

 spines in addition. The skull, of which only the ventral surface 

 is known, has a complete basis cranii, laterally expanded behind 

 by periotic capsules, and in front by what seem to be bulging 

 olfactory capsules. Anteriorly, the skull terminates in a ring of 

 calcified cirri. Behind the skull there are two singular post-occipital 

 plates, one on each side of the anterior section of the vertebral 

 column. The tail was apparently furnished with a fringing 

 caudal fin, supported dorsally by the long forked, neural spines, 

 and below by the much shorter haemal spines. There is no trace 



1 Traquair, Ann. Nat. Hist. (6) vi. 1890, p. 485 ; Proc. Itoy. Phys. Soc. Edinb. 

 xii. 1893, p. 87 ; ibid. p. 312 ; P.Z.S. 1897, p. 314 ; Bashford Dean, Trans. New 

 York Acad. Sci. xv. 1896, p. 101 ; Mem. New York Acad. Sci. ii. 1900, p. 1. 



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