42 EOCENE MOLLUSCA. 
Planulites, Lam., Gyroceras, Lituitus, Campulites, Desh. (Cyrtoceras, Gold.), Phragmo- 
ceras, Orthoceras, Actinoceras, Koleoceras, Portl., and Poterioceras, M‘Coy (Gom- 
phoceras, Sow.) 
Of these genera, the Nautilus only has been found above the secondary formations. 
The generic distinctions are taken chiefly from the position of the siphuncle, and 
the mode of convolution or the form of the shell. 
Genus 4. Nautitus—Gualtieri, Linneus. 
Ocranus; BrsipHites. De Mont. 
OmpuaLia. De Haan. 
Gen. desc. Animal; 4ody oblong, posteriorly rounded, and terminating in a slender 
membranaceous tube ; head above, with an ambulatory disc ; arms, nineteen (?) on each 
side ;* /abial tentaculiferous appendages, four, arranged round the mouth; tentacula of 
three kinds, viz. ophthalmic, lamellose, two on each side ; brachial, annulose, twenty on 
each side; /adia/, annulose, twenty-four on each side; the whole body contained in 
the last chamber of a large multilocular shell, and affixed by two lateral muscles. 
Shell; discoidal, spiral, multilocular, with simple walls; the whorls contiguous, 
the last covering the others; septa transverse, concave without, perforated in the disc, 
margins quite simple. 
Animal corpore oblongo, postice rotundato, tubo gracili membranaceo terminato ; capite 
supra disco ambulatorio ; brachiis utrinque novemdecem ; (?) appendicibus labialibus ten- 
taculiferis, guatuor, circum os dispositis ; tentaculis trium generum, quorum, ophthalmicis, 
lamellosis, utrinque duobus ; brachialibus, annulosis, utrinque viginti ; labialibus, annulosis 
utringue viginti quatuor ; toto corpore in camerd ultima teste magne multilocularis recon- 
dito et musculis duobus lateralibus affixo. 
Testa discoided, spirali, polythalamid, parietibus simplicibus ; anfractibus contiguis, 
ultimo alios obtegente ; septis transversis, extis concavis, disco perforatis, marginibus 
siemplicibus. 
The Nautilus is the only genus of the Cephalopoda which, appearing among the 
earliest forms of animal life, has survived the various changes which the earth has 
undergone. The large family, of which it forms the type, flourished during the 
Paleozoic epoch, and the Nautilus itself apparently attained its fullest development 
during the deposition of the carboniferous series, at which period nearly fifty species 
existed. Gradually diminishing in numbers, the genus passed through the Mesozoic 
epoch into the tertiary era, which it has also survived; and though reduced to four 
species, which have not any fossil representative,t it still exists in the tropical seas. 
* M. Valenciennes states the number to be seventeen. 
t The identification of the species in the Miocene formations of Turin cannot be relied upon. 
