﻿BEITISH FOSSIL CEPHALOPODA. 21 



exterior are repeated on the surface exposed in a more feeble manner, sometimes a 

 different set of structural markings appear. Since the exterior layer is a single 

 one — and from its mode of formation must be so — these lower surfaces must in all 

 cases belong to the inner or nacreous layer, and there is never any evidence to show 

 that the outer layer is double. 



Some of the shells of the Orthocerata, as such 0. primcevum, seem to have been 

 very thin ; while others, such as 0. annulatum, are of considerable thickness. When 

 the surface shows marks of coloration, we may be sure of the presence of the outer 

 layer ; but in the case of the thin shells there is no proof of its preservation. Cases 

 are often met with in which the shell has been broken and repaired during life — 

 and this may be seen also in recent Nautilus shells — the outer layer taking part 

 in the repairs ; but this does not prove that it had more than one layer. 



2. The External Form. 



The essential form of the Nautiloidean shells is that of a cone, that is, a solid 

 produced by the motion of a curve which remains similar to itself, but uniformly 

 enlarging in size as its centre, or some fixed point in it, moves along another curve. 

 The different forms will therefore arise, first, from the shape of the curve of 

 section ; secondly, from the rate of its increase ; and thirdly, from the shape of 

 the curve on which the fixed point moves. The effect of these may be considered 

 separately. 



(a.) The form of section.' — The particular shape will depend on the direction in 

 which the section is taken, and it is therefore always supposed to be taken in a plane 

 perpendicular to the curve of motion. When the shell is involute, as in the 

 Nautilus, the actual section is only part of the whole curve which would be formed 

 geometrically by the same law ; as, for example, the section of the recent Nautilus is 

 half an ellipse. The most common form of section as regards species is the circular, the 

 greater number belonging to the genus Orthoceras. The next common is the elliptic ; 

 the long axis of which may either lie in the plane of symmetry, when the section is 

 said to be direct, or perpendicular to it, when the section is said to be transverse. 

 The next most common form is the oval, derived from the directly elliptic by one 

 end being broader than the other; the broad end may lie either towards the outside 

 or towards the inside. Other less frequent forms are the triangular, chiefly met 

 with among open whorled forms; the subquadrate, mostly belonging to involute 

 shells; and the polygonal or irregular, due to the importance of longitudinal 

 ornaments. It seems to me to be entirely a matter of individual fancy whether the 

 more remarkable of these forms should be allowed to have a generic value, that is 

 whether such names as Gonioceras or Trigonoceras should be employed. The shape 

 of the section, though normally remaining similar, does not always do so, but 

 frequently changes in the neighbourhood of the aperture of adults— the circular 



