CESTRACIONTIDiE. 
245 
and one on each side—extending far downwards; inserted portion 
not definitely separated from the exserted portion; posterior face 
narrow, concave, bounded by two prominent longitudinal edges, each 
with a series of large recurved denticles immediately within. 
Though the spines of this genus were considered to be paired by 
T. Stock 1 , the fish is now shown to be a typical Cestraciont (Hybo- 
dont) by R. H. Traquair 2 . 
Tristychius arcuatus, Agassiz. 
1837. TristycJuus arcuatus, L. Agassiz, Poiss. Foss. vol. iii. p. 22, pi. i.a. 
figs. 9-11. ^ ft? 
1837. Ptychacanthus sublcevis, L. Agassiz, tom. cit. p. 23, pi. v. figs. 1-3 3 . 
1883. TristycJiius arcuatus, T. Stock, Ann. Mag. Nat. Hist. [5] vol. xii. 
p. 177, pi. vii. 
1888. Tristychius arcuatus, E. H. Traquair, Geol. Mag. [3] vol. v. p. 83. 
1889. Tristychius arcuatus , R. H. Traquair, loc. cit. [3] vol. vi. p. 27. 
Type. Dorsal fin-spine; Anderson’s College, Glasgow. 
The single described species 4 . 
Form. <$' Loc. Lower Carboniferous: Scotch Coal-field. 
42083. Imperfect spine; Calciferous Sandstones, Anstruther, Fife- 
shire. Purchased, 1870. 
To Tristychius have also been doubtfully assigned 5 the teeth from 
the Coal-Measures of "Wettin, Prussia, described under the names of 
Hybodus carbonarius , Giebel (Fauna Vorwelt, Fisehe, 1847, p. 313) 
and Germar (Yerstein. Steinkohlengeb. Wettin, 1849, p. 71, pi. xxix. 
fig. 5), and H. vicinalis , Giebel (Fauna Yorwelt, Fisehe, p. 313) and 
Germar (op. cit. p. 72, pi. xxix. figs. 6, 7). 
A number of small Carboniferous fossils, probably to be regarded 
as the dermal tubercles of some of the foregoing genera, have been 
described under the names of Petrodus , F. M‘Coy (Ann. Mag. Nat. 
Hist. [2] vol. ii. 1848, p. 13 2), Stemmatoclu s, St. John & Worthen 
(PalrHIinoTs, vol. vi. 1875, p. 328)Pand Styracodus, E. F. Germar 
(Yerstein. Steinkohlengeb. Wettin, 1849, p. 70). 
The bodies named Petrodus are conical and more or less circular 
in form, usually with a very thin root ; the exserted portion is 
deeply furrowed, with rough radiating ridges, and its height does 
1 Ann. Mag. Nat. Hist. [5] vol. xii. (1883), p. 188. 
2 Geol. Mag. [3] vol. v. (1888), p. 83. 
3 This is considered to be the spine of the adult by Dr. R. H. Traquair. 
4 The supposed T. fimbriatus, Stock (tom. cit. p. 177, pi. vii. fig. 1), is founded 
upon a spine of a distinct genus, now named Harp acanthus (R, H. Trauqair, 
Ann. Mag. Nat. Hist. [5] vol. xviii. p. 493). 
5 T. Stock, tom. cit. p. 183. 
