﻿32 BEITISH FOSSIL CEPHALOPODA. 



of the previous septa leave a furrow on the dorsal surface, which deepens anteriorly 

 beneath the succeeding septum (PI. II., fig. 2, b), and which is doubtless connected 

 with the longitudinal band observed in this position on the surface of the mantle. 

 This small angular lobe of the sutures has a totally different origin from those large 

 ones which characterise the Ammonitoidea, and must not be confounded with them. 

 As to the lobe on the ventral side, it is only found in those species with a ventral 

 siphuncle, and is due to the existence of the siphonal neck, to be immediately 

 described, and hence is also not to be confounded with a true lobe. One or other of 

 these confusions has given rise to the supposed genus Bactrites. 



The convexity of the septa is of course partly dependent on the sutures ; if these be 

 concave towards the aperture on the whole, as they are in the Nautiloidea, the septa 

 cannot very well be convex in that direction, unless a reversed inflation were given to 

 them, which is never the case. If corresponding points of the suture on opposite sides 

 of the plane of symmetry be joined by straight lines, a surface would_ be produced 

 having a ventro-dorsal convexity equal to that of the suture, but absolutely none in 

 a transverse direction. Such septa are not uncommon both among coiled shells and 

 uncoiled. When in the latter the sutures form nearly a plane curve, the septa are 

 almost flat, as in the Gomphocerata. The true convexity is that which occurs in a 

 transverse direction, being independent of the shape of the sutures. Among the 

 Orthocerata this forms a good specific character, and occasionally in other genera. 

 It is measured by the height of the septum divided by its transverse diameter. In 

 coiled shells the convexity is not uniform, and cannot well be made use of as a 

 character. In the recent Nautilus, for example, it is greatest where the suture is 

 convex to the aperture, so that, in spite of sigmoid sutures, in a central section the 

 septa are pretty uniformly concave. This latter feature is known to be characteristic 

 of the Nautiloids, the Ammonitoids having a larger part of the surface convex 

 towards the aperture. 



Besides the general convexity, there are two elevations seen on the hinder surface 

 of the septum of the recent Nautilus and of some others. The first and least important 

 is a small closed one in the dorsal region, in connection with the small lobe of the 

 septum there (PI. II., fig. 2, a) ; it is most conspicuous in the young, and gradually 

 fades away in the adult (fig. 2, c). It lies upon the furrow of the preceding chamber, 

 and must have been caused by the little elevation on the surface of the mantle which 

 produced the papilla in the body-chamber. It is this lobe which, becoming more con- 

 spicuous in the fossil Nautili, has given origin to the supposed genus Bisiphifes, it 

 having been mistaken for a second siphuncle. On the uncoiled Orthocerata it cannot 

 be said to be often observed, but in several instances it is present. It is indicated by 

 a line which projects most at each septum and then dies away again, and it has led 

 to such species as Orthoceras Steinhaueri of the Carboniferous rocks, which has a 

 siphuncle not marginal, being classed as Bactrites. It is perhaps an open question how 



