256 ORGANIC REMAINS. 



One or two species of nautilus are said to occur in the oolite, 

 but we have not seen any genuine nautilites in that kind of rock. 



Belemnites. Belemnite or Thunderbolt. This is one of the 

 most singular shells in existence, and having no recent analogue, 

 except perhaps a few microscopic shells, it is difficult to ascertain 

 with precision what it has been in the recent state, especially as it is 

 rarely, if ever, found entire. The chambers, instead of being dis- 

 posed spirally, are arranged one above another, so as to form a cone, 

 more or less tapering ; the septa being a series of cups, very small 

 towards the apex of the cone, and increasing in size as they approach 

 the base or mouth. These cups, like the septa of the recent nautilus, 

 have their hollow sides towards the mouth. In the last and largest 

 of them, which probably had some peculiar margin, the animal has 

 lodged, maintaining a communication with the other chambers, by a 

 siphuncle that pierces all the cups or septa, and has no doubt served, 

 like the siphuncle of the nautilus, to regulate the ascent and descent 

 of the animal in the water. The process has perhaps been facilitated 

 by a singular appendage belonging to the shell, enveloping the 

 conical part like a sheath, extending beyond it, in a cylindrical or 

 rounded form, and at last tapering to a point. This appendage, 

 which varies in shape, is always found in a sparry state, and the cross 

 fracture displays a number of lines radiating from a centre, like a star. 

 The centre of the star is not always in the middle, bvit often ap- 

 proaches to one side; being determined hyaline passing from the 

 apex of the chambered cone, to the exterior apex, or point of the 

 belemnite, which line is not always central. The part that envelops 

 the chambered cone is radiated from the surface of the cone outwards. 

 The exterior surface of the belemnite is generally smooth, and has 

 more the aspect of horn than of shell, especially as it is often trans- 

 lucent. Under cei'tain circumstances it acquires a rich silky or 

 pearly lustre. 



