266 
NAXJTILOIDEA. 
1833. Orthoceratites nautiloides, Steiainger, M^iu. Soc. G6ol. de France, 
vol. i. p. 369, pi. xxiii. ff. 1, 1 a. 
1837. Cyrtocera depressa^ Bronn, Lethiea Geognostica, 2nd ed. Band i . 
p. 101, tab. i. f. 5. 
1842. Cyrthoceratites depressus, d’Arcbiac and de Verneuil, Trans. 
Geol. Soc. 2nd ser. vol. vi. pt. ii. p. 350, tab. xxix. £f. 1, 1 a. 
? 1843. Cyrtoceras depresswn, Roemer, Die Verstein. des Harzgebirges, 
p. 35, tab. X. ff. 2, a, h. 
1849. Cyrtoceras nautiluideum, d’Orbigny, Prodr. de Pid^ont. Stratigr. 
vol. i. p. 53. 
1850. Cyrtoceras depressum, Roemer, Beitriige z. Geol. Kenntn. des 
Nordwestl. Harzgebirges, p. 38, tab. vi. f. 4. 
1854. Cyrtoceras depressum, Roemer, Palmontograpbica, Band iii. 
ibid. 
1883. Cranoceras depressmn, Hyatt, Proc. Boston, Soc. Nat. Hist, 
vol. xxii. — ‘‘Genera of Fossil Cepbalopods,” p. 281. 
Sp. Char. Shell large, rapidly expanding, forming half a volution. 
Section subtriangular, the transverse diameter wider than the 
ventro-dorsal, the ratio in an adult specimen being as 43 : 55. The 
body-chamber is short, and the aperture somewhat contracted h 
The septa increase gradually in their distance from each other, from 
2 ^ lines in the young shell to 5 lines in the adult ; they are but 
slightly concave, and their sutures bend forwards in crossing the 
convex or peripheral margin of the shell. The siphuncle increases 
considerably in diameter with the growth of the shell ; for example, 
where the dorso-ventral diameter is 2 inches 2 lines, the siphuncle 
is 3 lines in diameter, and where the dorso-ventral diameter of the 
shell has increased to 3 inches 8 lines, that of the siphuncle 
measures lines. It is filled with radiating deposits. “ Shell 
[test] very thick upon the sides, thinner upon the back, marked 
with fine close-set lines of growth, rising over the back where they 
cross the suture of the septa at an angle which increases in pro- 
portion as these latter are nearer to the aperture ” {d'Archiac and 
de Verneuil). Zigzag bands of colour are seen upon the cast in one 
of the specimens in the Museum Collection (Registered No. 66380) 
KemarTcs. In its rapid expansion, in the distance of its septa, and 
in the character and position of the siphuncle, the present species 
bears a considerable resemblance to the Silurian species C. Turnus, 
Barr. ® ; but the latter is much less coiled, and the siphuncle pro- 
portionately smaller. 
^ According to Prof. Hyatt, loc, cit. 
^ See also Cyrtoceras {Meloceras) Iridis, Barr. pi. cliii. 
^ Syst. Sil. de la Boheme, vol. ii. 1877, Suppl. et Ser. tardive, p. 46, 
pi. cccclxxxiii. f. 1, pi. cccclxxxiv. ff. 1, 2. 
