XI] OF SEPTA 583 



mathematically) that the sinuous or saddle-shaped form of the 

 "suture" (or line of attachment of the septum to the tube) is 

 such as can be precisely accounted for in this manner. It is also 

 easy to see that, when the section of the tube (or "generating 

 curve") is more complicated in form, when it is flattened, grooved, 

 or otherwise ornamented, the curvature of the septum and the 

 outhne of its sutural attachment will become very comphcated 

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

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

 of whorls has taken place, and this comparative simphcity of the 

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



We have other sources of comphcation, besides those which 

 are at once introduced by the sectional form of the tube. For 

 instance, the siphuncle, or httle inner tube which perforates the 

 septa, exercises a certain amount of tension, sometimes evidently 

 considerable, upon the latter; so that we can no longer consider 

 each septum as an isotropic surface, under uniform pressure ; and 

 there may be other structural modifications, or inequahties, in 

 that portion of the animal's body with which the septum is in 

 contact, and by which it is conformed. It is hardly hkely, for 

 all these reasons, that we shall ever attain to a full and particular 

 explanation of the septal surfaces and their sutural outlines 

 throughout the whole range of Cephalopod shells ; but in general 

 terms, the problem is probably not beyond the reach of mathe- 

 matical analysis. The problem might be approached experi- 

 mentally, after the manner of Plateau's experiments, by bending 



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

 ment the modem classification of the nautiloid and ammonitoid shells largely 

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

 xxvn, xxvra, 1829. 



f 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 fuUy formed, 

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

 matter. 



