44 EOCENE MOLLUSCA. 



which are bent forwards. The chambers are separated by transverse partitions, more 

 or less undulated ; and in one species, N. Parkinsoni, they are distinguished by lateral 

 angular lobes, resembling those of Aturia {Nautilus) zic-zac, and the margins are 

 invariably simple and entire. The discs of the septa are perforated at the centre, or 

 at parts more or less distant from the margins, but never at the margin, by a calcareous 

 siphuncle, variable in size and generally discontinuous, that is, extending more or less 

 into the preceding chamber, but not into the preceding siphuncular aperture. The 

 chambers themselves increase in size to the last, which is sufficiently large to contain 

 the whole of the animal ; but the ratio of increase is apparently uncertain, and is 

 influenced probably by the growth of the animal, which would, of course, depend on 

 the supply of food and other circumstances. 



The fossil substances termed Phyncolites, which occur so frequently in the older 

 formations, and which are generally believed to be the mandibles of some of the Tetra- 

 branchiate Cephalopods, with whose remains they are associated, have been found 

 both in the Paris basin and in the tertiary formations in Belgium ; but I believe that 

 as yet they have not been found in the Eocene strata of England. 



The specific characters in this genus are taken from the curvature of the septa, 

 the general outward form of the shell, (which, in fact, determines the shape of the 

 septum,) the position of the siphuncle and the condition of the umbilicus. With 

 respect to the terms dorsal and ventral, it must be borne in mind that they are used in 

 the following descriptions in a sense directly the reverse of that in which they have 

 been generally applied. The Nautilus, in its normal position, rests upon, or creeps 

 along the ground by means of, the free and expanded anterior portion of the mantle. 

 In this position the back of the animal is against the penultimate whorl of the shell, 

 and the ventral part is contained within the concavity of the dwelling-chamber. In 

 the following descriptions, therefore, the term dorsal is used to designate the parts 

 contiguous to the penultimate volution of the shell, and which have been generally, 

 though incorrectly, described as ventral ; and the term ventral, on the other hand, will 

 be applied to those parts on which the belly of the animal rested, and which hitherto 

 have usually been termed dorsal. 



At present six species have been found in the tertiary strata of England, and they 

 are confined to the older Eocene deposits. In the contemporaneous strata of the Paris 

 basin two species occur, one of which is also found in Belgium ; but not either of 

 them has as yet been found in England ; and four species have been described by 

 Sismonda and Michelotti, as occurring in the Miocene formations in Piedmont. Two 

 of these last species are referred by those authors to existing species ; but the 

 accuracy of the identification is questioned. 



