ACANTHOD TD-TE. 
11 
ornamented with about four longitudinal grooves. 
stouter and longer than the others 
slightly in advance of the dorsal, 
gonal striations. 
Pectoral spines 
pelvic spines small; anal spine 
Scales minute, with faint dia- 
Form. Sf 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. 
M M U - Li £ 
Acanthodes pusillus, Agassiz. 
[Plate I. figs. 5, 6.] 
1844. Acanthodes pusillus, L. Agassiz, Poiss. Foss. ^ . 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 (PI. 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. A Loc. Lower Old Red Sandstone : Banffshire, Scotland. 
35784-5- Two specimens, exhibiting the elongated upper caudal 
lobe : Tynet Burn. Purchased , l8 ,,n . 
