﻿ACANTHODIDiE. 11 



ornamented with about four longitudinal grooves. Pectoral spines 

 stouter and longer than the others ; pelvic spines small ; anal spine 

 slightly in advance of the dorsal. Scales minute, with faint dia- 

 gonal striations. 



Form. <$f Loc. Upper Devonian : Scaumenac Bay, P. Q,., Canada. 



Not represented in the Collection. 



The following small Devonian species have recently been assigned 

 by R. H. Traquair to a distinct genus, Mesacanthus, characterized 

 by the presence of a minute pair of free spines on the ventral sur- 

 face between the pectoral and pelvic fins. The proportions of the 

 paired fins are certainly somewhat different from those of the typical 

 Acanthodes, and decided points of generic distinctness may even- 

 tually be discovered. At present, however, we propose to retain 

 the long-accepted nomenclature ; more especially as the minute 

 additional pair of spines is not observable in any of the specimens 

 mentioned below, except a few examples of A. mitchelli. 



Acanthodes pusillus, Agassiz. 

 [Plate I. figs. 5, 6.] 



1844. Acanthodes pusillus, L. Agassiz, Poiss. Foss. V. G. R. p. 36, 



pi. xxviii. figs. 8-10. 

 1888. Mesacanthus pusillus, R. H. Traquair, Geol. Mag. [3] vol. v. 



p. 512. 



Type. Imperfect fish ; Forres Museum. 



A very small species, the largest specimen in the Collection (PL I. 

 fig. 5) measuring not more than 0*06 in extreme length. Body 

 elongated and slender, the maximum depth being contained about 

 six or seven times in the total length ; caudal lobe extremely 

 elongated. Pelvic fins large, midway between the pectorals and the 

 anal ; pelvic spines not less than half the size of the pectorals, and 

 two thirds as long as the anal spine. Dorsal fin arising behind the 

 origin of the anal, slightly larger than the latter. 



In the original description of A. pusillus, Agassiz mentions the 

 presence of a series of small spines upon the lower border of the 

 caudal region. The character, however, is not alluded to by Egerton 

 when comparing this species with A. peachi, and it is not exhibited 

 by any of the specimens enumerated below. 



Form. Sf Loc. Lower Old Red Sandstone : Banffshire, Scotland. 



35784-5. Two specimens, exhibiting the elongated upper caudal 

 lobe ; Tynet Burn. Purchased, 1860. 



