190 MANUAL OF THE MOLLUSCA. 
ig at bale _- 
Fig. 52. Clymenia striata, Munst.* Fig. 53. C. linearis, Munst. 
CLYMENIA, Munster, 1832. 
Etymology, Clymene, a sea-nymph. 
Synonyms, Endosiphonites, Ansted. Sub-clymenia, D’Orb. 
Example, C. striata, Pl. II., Fig. 16 (Mus. Tennant). 
‘Shell discoidal; septa simple or slightly lobed; siphuncle 
internal. 
Fossil, 45 species. Upper Silurian—Mount. Limestone. 
North America, Europe. 
FaAamity II.—ORTHOCERATIDA. 
Shell straight, curved, or discoidal; body chamber small; 
aperture contracted, sometimes extremely narrow (Figs. 48, 
49); siphuncle complicated. 
It seems probable that the cephalopods of this family were 
not able to withdraw themselves completely into their shells, 
like the pearly nautilus; this was certainly the case with some 
of them, as M. Barrande has stated, for the siphonal aperture 
is almost isolated from the cephalicopening. The shell appears 
to have been often less calcified, but connected with more 
vascular parts than in the nautilus; and the siphuncle often 
attains an enormous development. In all this, there is nothing 
to suggest a doubt of their being tetrabranchiate ; and the cheyron- 
shaped coloured bands preserved on the orthoceras anguliferus,} 
sufficiently prove that the shell was essentially external. 
ORTHOCERAS, Breyn. 
Hiymology, orthos, straight, and ceras, a horn. 
Synonyms, Cycloceras, McCoy. Gonioceras, Hall.t Conoceras, — 
Bronn. 
Example, O. Ludense (diagram of a longitudinal section) 
Pit. Bae. 14. 
Shell straight; siphuncle central; aperture sometimes con- 
tracted. 
Fossil, 240 species. Lower Silurian—Lias; North America, 
Australia, and Europe. 
* Figs. 52,53. Sutures of two species of Clymenia from Phillips’ Pal. Fos., Devon- 
shire. + Figured by D’Archiac and Verneuil, Geol. Trans. 
t Zheca and Tentaculites are provisionally placed with the Pteropoda: they pro 
bably belong here. 
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