﻿ACANTHODID^. O 



The squamation appears to be of equal fineness over the whole of 

 the trunk, there being no fulcral scales even on the upper caudal 

 lobe ; while the only diminution in the size of the quadrate granules 

 occurs in this region and towards the distal margins of the fins. 

 The scales are either rectangular or slightly rhomboidal, with a flat- 

 tened or faintly excavated external surface, usually smooth ; and 

 their attached surface exhibits a gentle convexity. A single lateral 

 line occurs high on each flank, marked not by any tubular or other 

 excavation of the scales, but by the ridge-like displacement of two 

 series, between which the organ originally extended. The supposed 

 evidence of additional sensory canals appears to the present writer 

 to be due to a misinterpretation of the displaced dorsal and ventral 

 ridges, which exhibit no median series of scales. 



The evolution of the paired fins in the successive species of Acan- 

 thodes, as defined below, is of some interest. In the Upper Devonian 

 representatives of the genus the pelvic fins are not much inferior in 

 size to the pectorals, and are placed nearly midway between the 

 latter and the anal. In the Lower Carboniferous A. nitidus the 

 pelvic fins are similarly placed, but reduced in size. In the Upper 

 Carboniferous A. wardi the same fins are not only further reduced, 

 but occupy a more forward position, while the pectorals are much 

 enlarged. In the Lower Permian species the pelvic fins become 

 insignificant and the pectoral fins enormous, while the two pairs are 

 even more closely approximated than in the earlier forms. 



Acanthodes bronni, Agassiz. 



1832. Acanthoessus bronnii, L. Agassiz, Neues Jahrb. p. 149. 

 1833-35. Acanthodes bronnii, L. Agassiz, Poiss. Foss. vol. ii. pt. i. 



pp. 20, 124, P l. i. 

 1848. Holacanthodes gracilis, E. Beyrich, Monatsb. Beri. Akad. p. 24. 

 1857. Acanthodes gracilis, F. Roemer, Zeitschr. deutsch. geol. Ges. 



vol. ix. p. 65, pl. iii. 

 1857. Acanthodes bronnii, F. H. Troschel, Verhandl. naturh. Ver. preuss. 



Rheinl. u. Westph. vol. xiv. p. 2, pis. i., ii. figs. 1-13. 

 1861. Acanthodes gracilis, H. B. Geinitz, Dyas, p. 21. 

 1864. Acanthodes bronnii, E. Weiss, Zeitschr. deutsch. geol. Ges. vol. 



xvi. p. 291. 

 1868. Acanthodes bronnii and A. gracilis, R. Kner, Sitzungsb. k. k. 



Akad. Wiss. Wien, math.-naturw. CI. vol. lvii. pt. i. p. 303, pis. 



v.-viii. 



Type. Imperfect fish; olim H. G. Brown Collection. 



The type species, attaining a maximum length of about 0*3. 

 Body much elongated and slender, the maximum depth being con- 

 tained about six or seven times in the total length. Pectoral fin- 



i 



