190 MANUAL OF THE MOLLUSC A. 



Fig. 52. Clymenia striata, Munst.* Fig. 53. C. linearis, Munst. 



Olymenia, Munster, 1832. 



Etymology, Clymene, a sea-nymph. 



Synonyms, Endosiplioiiites, Ansted. Sub-clymenia, D'Orb. 



Example, C. striata, PI. II., Fig. 16 (Mus. Tennant). 



Shell discoidal; septa simple or slightly lobed ; siphuncle 

 internal. 



Fossil, 45 species. Upper Silurian — Mount. Limestone. 

 North America, Europe. 



Family II. — ORTHocEEATiDiE. 



Shell straight, curved, or discoidal; body chamher 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 cephalic opening. 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 tetrahranchiate ; and the chevron- 

 shaped coloured bands preserved on the orthoceras anguliferus,-\ 

 sufficiently prove that the shell was essentially external. 



Orthoceras, Breyn. 



Etymology, orthos, straight, and ceras, a horn. 



Synonyms, Cycloceras, McCoy. Gonioceras, Hall.:{: Conoceras, 

 Br^nn. 



Example, 0. Ludense (diagram of a longitudinal section) 

 PI. II., Fig. 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- 

 Bhire. t Figured by D'Archiac and Verneuil, Geol. Trans. 



t Iheca and Tentaculites are provisionally placed with the JPteropoda : they pro- 

 bably belong here. 



