CHAPTER XxX 
APPENDIX TO THE FISHES: PALAKOSPONDYLIDAE—OSTRACODERMI 
— HETEROSTRACI— OSTEOSTRACI — ANASPIDA—— ANTIARCHI — 
ARTHRODIRA. 
In 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,’ 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, 
ealcified 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. Roy. 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. 11. 1900, p. 1. 
521 
