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. Nautilus — Gualtieri, Linnceus. 

 Oceanus ; Bisiphites. Be Mont. 

 Omphalia. Be Haan. 



Gen. desc. Animal ; body oblong, posteriorly rounded, and terminating in a slender 

 membranaceous tube; head above, with an ambulatory disc ; arms, nineteen (?) on each 

 side ;* labial tentaculiferous appendages, four, arranged round the mouth ; tentacula of 

 three kinds, viz. ophthalmic, lamellose, two on each side ; brachial, annulose, twenty on 

 each side ; labial, 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, quatuor, circum os dispositis ; tentaculis trium generum, quorum, ophthalmicis, 

 lamellosis, utrinque duobus ; brachialibus, annulosis, utrinque viginti ; labialibus, annidosis 

 utrinque viginti quatuor ; toto corpore in camera ultima testa magnce multilocularis recon- 

 dito et musculis duobus lateralibus affixo. 



Testa discoided, spirali, poly thalamic!, parietibus simplicibus ; anfractibus contiguis, 

 ultimo alios obtegente ; septis transversis, extus concavis, disco perforatis, marginibus 

 simplicibus. 



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 

 Palaeozoic 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,! it still exists in the tropical seas. 



* M. Valenciennes states the number to he seventeen. 



t The identification of the species in the Miocene formations of Turin cannot be relied upon. 



