174 
NAUTILOIDEA. 
1849. Nautilus cyclostomus, d’Orbignj, Prodr. de Paleont. Stratigr. 
vol. i. p. 111. 
1852. Asymptoceras cyclostomus, Pyckholt, Notice sur les genres 
Nautilus, Vestinautilus, &c. p. 6. 
1854. Nautilus cyclostomus, Moms, Cat. of British Fossils, p. 308. 
1860. Nautilus cyclostomus, Eichwald, Lethaea Rossica, Seconde Sec- 
tion de I’ancienne P^riode, vol. i. p. 1317. 
1860. Nautilus cyclostomus, Griffith, Journ. Geol. Soc. Dublin, vol. ix. 
p. 56. 
1878. Nautilus cyclostomus, de Koninck, Faime du Calcaire Carboni- 
fere de la Belgique (Annales du Mus. Boy. d’Hist. Nat. de Bel- 
gique, tom. ii.), p. 112, pi. xxiii. ff. 1 a, 1 6, 2 a, 2 5, 2 c. 
Sp. Char. Shell Nautilus-like, composed of three rapidly ex- 
panding whorls, which are only just in contact, and ail exposed in 
a moderately deep umbilicus, which has a small central perforation. 
The initial point is obtuse at its extremity ; no cicatrix has been 
seen upon it. The transverse section is circular. The body- 
chamber is large and occupies half of the last whorl ; it is prolonged 
for a short distance in adult shells beyond the coiled part. The 
peripheral border of the aperture is very distinctly emarginate. 
The septa are tolerably numerous, and consequently not very dis- 
tant from each other, numbering twenty-one or twenty-two in a 
complete whorl. The siphuncle is situated on the peripheral mar- 
gin, and is nearly always of a black colour. The test is extremely 
thin in all parts of the shell. The ornaments upon its surface in 
the young shell consist of fine longitudinal thread-like lines, which 
almost completely disappear when the second whorl is reached, 
being longest persistent on the ventral side. Lines of growth are 
also met with, chiefly in the region of the aperture. 
Remarks. This species is easily distinguished by the circular form 
of its section, its non-enveloping whorls, and the ornamentation of 
the test. 
One of the specimens (a fragment of a body-chamber) figured by 
Phillips (Geol. of Yorkshire, pi. xxi. f. 13) is doubtfully referred by 
him to the present species, and is described only in the explanation 
of pi. xxi. thus: — “Part of Nautilus cyclostomus '1^’ It most pro- 
bably belongs to another species, the siphuncle being situated half- 
way between the centre and the peripheral margin, instead of being 
quite at the edge of the periphery, as in S, cyclostomus. 
S. cyclostomus has been found in Ireland, England (Yorkshire), 
Russia (IJral Mts.), and Belgium. 
Horizon. Carboniferous Limestone. Calcaire Carbonifere supe- 
rieur, Assise vi. (Belgium). 
