82 PALEONTOLOGY 



siderable motion of the animal in its shell. When the outer 

 shell of the fossil is removed by decomposition or the ham- 

 mer, the margins of the internal septa (or partitions of the 

 air-chambers) are exposed : these marginal lines are called 

 " sutures." 



The chambered shells may be divided into two principal 

 groups, viz., those with simple sutures, like the recent Nauti- 

 lus ; and those in which the margins of the septa are lobed 

 and foliaceous, the Ammonites. In the former the siplmncle 

 is central or internal (i.e., at the margin next the spire) ; in 

 the latter it is external (i.e., at the back of the shell, but ventral 

 as regards the animal). There are, however, Nautili with lobed 

 sutures (Aturia, Bronn, fig. 19, i) ; and some with an exter- 

 nal siphuncle (Cryptoceras, d'Orb.) And on the other hand, 

 the sutures of the Ammonite are at first very slightly lobed, 

 and become progressively more complex ; so that specimens 

 of the same species have been referred to three genera — Goni- 

 atitcs, Ceratites, and Ammonites — according to their age. 



With the exception of Goniatites, the Ammonitidce are 

 peculiar to, and co-extensive with, the secondary strata ; while 

 the Naidilidm, with the exception of Nautilus and Aturia, are 

 confined to the palaeozoic rocks. But the palaeozoic so-called 

 Naidilidm exhibit peculiarities suggesting very wide differences 

 from the modern pearly Nautilus. It has been proposed to 

 associate the greater part of them with the Orthocerata as a 

 distinct family, but at present the data are defective. Like 

 the Ammonitidw, their shells assume almost every conceivable 

 form and curvature, and the genera foimded on these charac- 

 ters are very ill defined. 



The simplest form of Orthoccras is like a Nautilus unrolled ; 

 and Lituites (fig. 20, 2) is the same with the apex spiral. Some 

 of the carboniferous Nautili have a square back, and the whirls 

 either compact or open in the centre (fig. 20, 1) ; whilst the 

 last chamber is more or less disunited. The species with the 



