﻿30 BEITISH FOSSIL CEPHALOPODA. 



whole ornament slopes downwards or upwards till it meets the next : these are 

 called upward or downward imbrications respectively. 



7. The Septa. 



In the Nautiloidea these are comparatively simple, but they give rise, neverthe- 

 less, to several points of interest. 



(a.) Their distance apart. — The first septum begins at the junction of the cap 

 with the normal spiral (PI. II., fig. 4), and cuts off the former. The first apparent 

 chamber is therefore different from all the rest, and its relative size depends entirely 

 on the shape of the nucleus, and this, in different species, may be either much greater 

 or much less^than the second chamber. Starting from the first septum, we do not 

 find them at distances constantly proportional to the diameter. If they were, there 

 would always be the same number in each whorl. In an example, however, of 

 Nautilus pompilius, the first whorl has eight chambers, the next has sixteen, and 

 the last half whorl seven. The same want of regularity is found in other Nautiloids. 

 As a rule, the earlier septa are more remote, and the middle ones only retain for 

 some distance their proportionality. The last two or three septa of the adult very 

 commonly differ in distance from the rest. In some rare cases they are more remote, 

 but they are usually closer, and the commonest case is that in which the last one is at 

 half the usual distance. This is doubtless to be regarded as a proof of senility, and 

 the fading of the powers which caused the animal's progression, which was more 

 rapid in youth. Some caution is therefore required in using the distance of the 

 septa to recognise fragments by ; but when a complete specimen is before us, it 

 affords a valuable specific character. The Gornphoceratidce and the Silurian Cyrto- 

 cerata have usually remarkably close septa. The space between the septa may be 

 expressed either as a fraction of the diameter of the whorl, or the number in any 

 given whorl may be stated. 1 



(b.) Their general direction. — In those of more complicated form we can only 

 express this by a line joining their central point in the ventral to that on the dorsal 

 side. If this passes through the pole of the curve of motion, the general direction 

 may be said to be radial ; if not, they have a general slope forwards or backwards. 

 Radial septa will of course make the same angle with the general direction of the 

 whorl as the spiral angle of the curve, and are in one sense oblique. In slightly 

 curved shells, such as Cyrtocerata, it is more convenient to call them direct when 

 they are perpendicular to the general direction, whether radial or not, and the same 

 terminology applies to Orthocerata, &c. The amount of obliquity varies from one 



1 These are connected as follows approximately. If r be the ratio of the distance of the septa to 



the breadth of the whorl, n the number of chambers per half whorl, d the diameter of the whorl, and 



u the diameter of the umbilicus, 



7T d-\-u 



nr = /r- • 3 



2 a - u 



