140 
ICHTH YODOR CXITES. 
teriorly. The longitudinal mesial line of the anterior face, except 
near the unworn apex, defined only by the superficial ornament, 
which consists of parallel, oblique, transverse ridges, diverging in 
pairs from this line and inclined towards the inserted extremity ; 
posterior face with a narrow unornamented area, sometimes bounded 
by a series of denticles on one side; unworn apex also destitute of 
ornament. Free spines broad, laterally compressed, usually triangular, 
sometimes of reniform shape ; base-line straight on one side, much 
excavated on the other, the central cavity very large and its walls 
thin ; exserted portion having a rough fibrous appearance, usually 
with a few rounded tubercles at the distal pointed end, these some¬ 
times exhibiting a tendency to arrangement in transverse series. 
The paired spines have been described in detail by E. H. Traquair 1 , 
who points out that no known British specimens are bilaterally 
symmetrical. Notwithstanding this peculiarity, Agassiz supposed 
that they might have armed the dorsal fins; and Kirkby and 
Atthey 2 seem to have been the first to suggest their pertaining to 
paired fins. Hancock and Atthey, in 1868 3 , considered that a few of 
the spines exhibited true bilateral symmetry, and might thus be 
median dorsal; but Traquair regards these as the paired spines of 
young individuals, those of more mature individuals being much 
altered in appearance by the continual abrasion of the apex. 
The free spines are usually found in intimate association with 
the fin-spines, and they were thus originally described by Hancock 
and Atthey i 5 as “ carpal bones ” (i. e. basal cartilages of the pectoral 
fins); an examination of microscopical sections, however, has 
demonstrated their truly dermal nature 3 . 
Pectinated shagreen-granules are also met with in association 
with the spines of Gyracanthus, and microscopical sections of these 
have been described under the name of Mitroclus quadricornis , 
Owen 6 . 
Gyracanthus formosus 9 Agassiz. 
1825-26. “ Fish spine P,” J. de C. Sowerby, Zool. Journ. vol. i. p. 2*52, 
pi. viii. fig. 9, and ibid. vol. ii. p. 22. 
1837. Gyracanthus formosus, L. Agassiz, Poiss. Foss. vol. iii. p. 17, 
pi. v.'figs. 2-6. 
1 Ann. Mag. Nat. Hist. [5] vol. xiii. (1884), p. 38. 
2 According to Hancock and Atthey, 1868. 
3 Ann. Mag. Nat. Hist. [4] vol. i. (1868), p. 368. 
4 Ann. Mag. Nat. Hist. [4] vol. i. (1868), p. 369, and ibid. vol. is. (1872), 
p. 260. 
5 R. H. Traquair. Ann. Mag. Nat. Hist. [5] vol. siii. (1884), p. 44. 
e R, Owen, Trans. Odontol. Soc. vol. v. (1867), p. 338, pi. iii. 
