nautilidj:. 
71 
the ventral side being hollowed out and bordered by prominent 
ridges, the sides gently convex and sloping gradually inwards till 
they meet in a sharp ridge in the median line of the concave side. 
The septa are moderately distant and form a deep sinus on the 
ventral side of the shell. The siphuncle is situated a little above 
the centre. The test is thin, its ornaments consist of distant, longi- 
tudinal ridges, which are very faint upon the sides, but are stronger 
upon the superior lateral angles, where there are three or four of 
them close together; there is a prominent ridge also along the 
median line of the concave side of the shell. The test has very fine 
close-set longitudinal lines which can only be seen with the aid of a 
J^ms. 
This subgenus, so far as is known, is confined to the Carboniferous 
rocks of Ireland and Belgium. It is represented only by the two 
species described below. 
llemarlcs. Some palaeontologists — Giebel, Barrande, Blake, &c. — 
regard Triyonoceras as allied to, or even as a subgenus of, Cyrtoceras, 
but its scutiform section and whorled apex indicate its affinities 
rather with Gyroceras, With the latter it has been associated by 
de Koninck ; but the fragment upon which he based his decision 
(evidently conspecific with Sowerby’s 0. 2^arm^.rhao»)has a restored 
outline added to it, making \h volutions, for which there is no 
warrant in any of the specimens described by Sowerby and W‘Coy, 
nor in those contained in the British Museum. 
Gyroceras (Trigonoceras)paradoxicum,J.de C. Sowerby, sp. 
1825. Orihocera jmrado.vica, J. de C. Sowerby, Min. Conch, vol. v. 
p. 81, pi. cccclvii. 
1830. Orthoceras paradoxicum, Phillips, Geol. of Yorkshire, pt. ii. 
p. 239. 
1844. Orthoceras {Triyonoceras) paradoxicum, M^Coy, Synop. of the 
Carboniferous lamestone Foss, of Ireland, p. 9. 
1849. Aploceras paradoxicum, d'Orbigny, Prodr. de PalcSont. vol. i. 
p. 112. 
1852. Cyrioceras paradoxicum, Giebel, Die Fauna derVorvvelt, Band iii. 
Abth. i. p. 200. 
1 855. Orthoceras ( Triyonoceras) paradoxicum, M‘Coy, British Pal. F oss. 
fasc. iii. p. 573. 
1880. Gyroceras paradoxicum, de Koninck, Faime du Calcaire Carboni- 
fere de la Belgique (Annales du Mus. Roy. dTIist. Nat. de Belgique, 
tom. V.), p. 7, pi. xx.xii. ff. 3, 3 3 h. 
8p. Char. “ Rapidly tapering, abruptly incurved towards the 
smaller end, forming about half a volution ; external face gently 
concave, bounded by strong angles (slightly less than rectangular) ; 
