262 
NAT7TIL0IDEA. 
Family CYRTOCERATID^E. 
Genus CYRTOCERAS, Goldfuss\ 
{Amimonus, Montfort, 1808 ^ ; Conilites (pars), Blainville, 1825 ^ ; Or- 
thocera (pars), Fleming, 1828 ^ ; Campulites, Deshayes, 1832 ® ; Orthoce- 
ratitesy Steiniager, 1833 ® ; Lituites (pars), Quenstedt, 1836 ^ ; Phragmo- 
ceras (pars), J. de C. Sowerby, 1839'^ ; ? Phragmoceras, Portlock, 1843^; 
Campy locerasj M^Coy, 1844^°; Aploceras, d’Orbigny, 1849 ? Phrag- 
moceras, Scbmidt, 1858 Phragmoceras (pars), Eicbwald, 1800 ; Cra- 
noceras^ Hyatt, 1883 \ \_Mcelonoceras and Oonoceras^ Hyatt are sub- 
genera].) 
Gen. Char. Shell more or less curved, but never forming a complete 
volution ; tapering rapidly (brevicone forms) or more slowly (longi- 
cone forms), these variations being illustrated in the subgeniis Mtlo- 
ceras. Or, on the other hand, the shell may increase so slowly that 
it may be described as almost cylindroid (subgenus Ooceras). Cross 
section usually ovate, the longer axis in the plane of curvature ; some- 
times subcircular or even circular, more rarely transversely elliptical 
and depressed. Siphuncle usually small, sometimes with radiating 
deposits {C. depressum, d’Arch. & de Vern. e. g.) ; either external 
(exogastric)., internal (endogastric), or subcentral {mediogastricY^ ; 
cylindrical or nummuloidal. Body-chamber generally large in pro- 
portion to the entire shell. Septa usually approximate, the last two 
often closer together than the rest. Ornamentation in the Ordovi- 
' Von Dechen’s Handb. der Geognosie von H. T. De la Beebe, 1832, p. 536 
(footnote). 
^ Conchyl. Systemat. vol. i. p. 327. 
® Manuel de Malacologie, p. 378. 
^ Hist, of British Animals, p. 239. 
® Encyclop. Methodique, vol. iii. p. 671. 
® Mem. Soc. Geol. de Erance, vol. i. pt. i. p. 368. 
De notis Nautilearum primariis, p. 23. 
® March. Sil. Syst. pt. ii. p. 621. 
® Geol. of Londonderry, pp. 382, 383. 
^0 Synop. Garb. Foss. Ireland, p. 6. 
Prodr. de Paltent. Stratigr. vol. i. p. 112. 
Silur. Form, von Ehstland, N.-Livland u. Oesel, p. 200. 
Lethaea Eossica, vol. i. p. 1274 et seq. 
Proc. Boston Soc. jMat. Hist. vol. xxii. p. 281. 
Idid. p. 280. These names shoiild be written Mehoeras and Ooceras, in 
which amended form I shall employ them. Meloceras is derived from yriXov, 
a goat ; Ooceras from i^ov, an egg. 
A division proposed by Blake to include species “ in which the siphuncle 
is near the centre ” (Brit. Foss. Ceph. pt. i. p. 56). 
