XI] THE SEVTA 0¥ NAUTILUS 845 



form of a logarithmic spiral but in any transverse section is a straight 

 horizontal line. If we shear or slide the cards upon one. another, 

 thrusting the middle cards of the pack forward in advance of the 

 others, till the one end of the pack is a convex, and the other a 

 concave, elUpse, the cut edges which combine to represent our 

 septum will now form a curved surface of much greater complexity ; 

 and this is part, but not by any means all, of the deformation 

 produced as a direct consequence of the form in Nautilus of the 

 section of the tube within which the septum has to He. The 

 complex curvature of the surface will be manifested in a sinuous 

 outline of the edge, or line of attachment of the septum to the tube, 

 and will vary according to the configuration of the latter. In the 

 case of Nautilus, it is easy to shew empirically (though not perhaps 

 easy to demonstrate mathematically), that the sinuous or saddle- 

 shaped contour of the " suture" (or line of attachment of the septum 

 to the tube) is such as can be precisely accounted for in this manner ; 

 and we may find other forms, such as Ceratites, where the septal 

 outline is only a httle more sinuous, and still precisely analogous 

 to that of Nautilus. It is also easy to see that, when the section of 

 the tube (or "generating curve") is more comphcated in form, when 

 it is flattened, grooved, or otherwise ornamented, the curvature of the 

 septum and the outline of its sutural attachment will become very 

 complicated indeed * ; but it will be comparatively simple in the "case 

 of the first few sutures of the young shell, laid down before any over- 

 lapping of whorls has taken place, and this comparative simplicity of 

 the first-formed sutures is a marked feature among Ammonites f. 



* The "lobes" and "saddles" which arise in this manner, and on whose arrange- 

 ment the modern classification of the nautiloid and ammonitoid shells largely 

 depends, were first recognised and named by Leopold von Buch, Ann. Sci. Nat. 

 XXVII, xxvni, 1829. 



t Blake has remarked upon the fact (op. cit. p. 248) that in some Cyrtocerata 

 we may have a curved shell in which the ornaments approximately run at a constant 

 angular distance from the pole, while the septa approximate to a radial direction; 

 and that "thus one law of growth is illustrated by the inside, and another by the 

 outside." In this there is nothing at which we need wonder. It is merely a case 

 where the generating curve is set very obliquely to the axis of the shell ; but where 

 the septa, which have no necessary relation to the mouth of the shell, take their 

 places, as usual, at a certain definite' angle to the walls of the tube. This relation 

 of the septa to the walls of the tube arises after the tube itself is fully formed, 

 and the obliquity of growth of the open end of the tube has no relation to the 

 matter. 



