169 



others ; the chambers separat^ed by loinding septa, pierced 

 by a siphuncle at the outer side of their margin. Fossil. 



The shells of this genus agreeing in their external form 

 with that of the shells of the preceding genus, nautilus, the 

 difference being internal, from the septa being winding, and 

 the siphuncle marginal, an illustrative figure was unnecessary. 



The propriety of the adoption of this genus seems to be 

 indisputable, its shells possessing the external characters of 

 9iavtUus, and the internal characters, the winding septa and 

 marginal siphuncle, of ammonites. But there are other 

 fossil shells, which, possessing the exterior of nautilus, have 

 only one of the characteristics of ammonites, as in Organic 

 Ilemains, PL vii. fig. 15, in which the winding septa are 

 without the marginal siphuncle : and in fig. 16 of the same 

 plate, where the marginal siphuncle exists without the 

 winding septa. Indeed, the situation of the siphuncle is 

 too variable, existing in every point on a line from the 

 centre, and, perhaps, from the posterior to the anterior 

 margin, to allow of its being assumed alone as a distinctive 

 generic character. 



3. Planulites. — A multilocular, spiral, flattish, and dis- 

 coidal shell ; the turns contiguous and apparent ; the cham- 

 bers separated by plain septa, pierced with a marginal 

 siphuncle. Fossil. 



As the shells of the preceding genus have possessed the 

 outer appearance of nautilus, with the internal characters 

 of ammonites, so this shell possesses the external appear- 

 ance oi ammonites, and, at the same time, is characterized 

 by the plain concave septa of nautilus. As its external 

 form agrees with that of the following genus, another figure 

 is not requisite. 



4. Ammonites. — A multilocular, spiral, and discoidal 

 shell, with contiguous apparent turns ; the chambers divided 

 by winding septa, pierced by a siphuncle always placed at 

 the outer side. Fossil. — PI. vi. fig. 2. 



z. 



