﻿140 ICHTHTODOR ELITES. 



teriorly. The longitudinal mesial line of the anterior face, except 

 near the unworn apex, denned 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 \ 

 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 4 as " carpal bones " (i. e. basal cartilages of the pectoral 

 fins) ; an examination of microscopical sections, however, has 

 demonstrated their truly dermal nature 5 . 



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 Mitrodus quadricornis, 

 Owen 6 . 



Gyracanthus formosus, Agassiz. 



1825-26. " Fish spine ?," J. de C. Sowerby, Zool. Journ. vol. i. p. 252, 



pi. viii. fig. 9, and ibid. vol. ii. p. 22. 

 1837. Gyracanthus fm-mosus, 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. 



< Ann. Mag. Nat. Hist, [4] vol. i. (1868), p. 369, and ibid. vol. ix. (1872), 

 p. 260. 



5 R, H. Traquair, Ann. Mag. Nat. Hiat. [5] vol. xiii. (1884), p. 44. 



6 R. Owen, Trans. Odontol. Soc. vol. v. (1867), p. 338, pi. iii. 



