﻿BRITISH FOSSIL CEPHALOPODA. 31 



species to another so continuously, that those with oblique septa are not marked off 

 as a group from the rest, and such a genus as Loxoceras, proposed by M'Coy, is not 

 therefore required. If there be ornaments on the shell, it is very usual for the septa 

 to have the same direction as they have ; but this is by no means universal, and 

 affords a good specific character. 



(c.) Their shape. — This may be considered dependent on two elements, viz. the 

 sutures, or their intersection with the surface of the shell, and the convexity, or 

 elevations and depressions on the surface of the septa, bounded by the sutures. 



The sutures form plane curves only in uncoiled shells, and even in these they 

 are more commonly concave towards the aperture. If the concavity does not extend 

 from one side to the other, they are called undulating, because they have a wavy- 

 like appearance ; the undulation may be either on the sides or on the ventral and 

 dorsal regions. In coiled shells the sutures are perhaps always more or less curved. 

 In the simplest cases the septum shows a single concavity towards the aperture, such 

 as might be produced by the intersection of a sphere with the surface of the shell. 

 When the latter is in any degree angular, like the Discites of the Carboniferous 

 rocks, or the subquadrate Silurian forms, it throws the sutures into corresponding 

 angles, which are of quite a different character, therefore, to the angles which in 

 other cases they make on perfectly uniform surfaces. From this we pass to the 

 form exhibited by the recent Nautilus, and many fossil ones which have sigmoid 

 sutures. In some the inner part of the suture is concave and the outer one convex, 

 or direct, but in others the arrangement is reversed. Certain of the Nautiloids have 

 very exaggerated sigmoid sutures ; while an anomalous group among the Clymenidce 

 have actually an angular lobe on the side. From the important developments which 

 take place in this direction when we enter the Ammonitoids, it might be thought 

 that such Clymenidce ought to be severed from the Nautiloids ; but there are no other 

 features to separate them, and a close approach to their form of suture may be found 

 among undoubted Nautili, as in Nautilus Parkinsoni from the London Clay. But the 

 most extraordinary departure from the simple form met with among the Nautiloids 

 is in Ascoceras. In these the earlier septa are normal, but the last few adhere 

 together in the centre and form but one line ; while on one side they are thrown into 

 deep sigmoids with the inner ends joining, and on the other are the short remainders 

 containing the siphuncle, as will be shown at greater length under the description 

 of this genus. 



Besides the wide curves of the sutures, there are found in some Nautiloids a 

 minute lobe on the dorsal and one on the ventral region. The former may be 

 admirably seen in the recent Nautilus. The dorsal part of the last septum, instead 

 of going straight across, is seen to be deflected backwards in the centre so as to form 

 a small angular lobe only a line in length. This lobe is occupied by a prominence 

 which has a circular depression in the middle, and a mulberry mass in front. Each 



