﻿40 BRITISH FOSSIL CEPHALOPODA. 



9 . The Internal Marks. 



There are several curious marks on different parts of the shell of Nautiloids 

 whose nature is in some instances unknown, though in all probability of importance 

 as connected with the organisation of the animal. These are — 



(a.) Vascular marks on the septa. — These always lie on the concave side ; and may 

 be easily seen in the last chamber of Nautilus pompilius (PI. II., fig. 3). They are 

 very shallow, and hence may be easily overlooked. They represent the impressions 

 of circulating vessels in the upper part of the mantle, and ramify from the neighbour- 

 hood of the siphuncle, and bifurcate towards the circumference. Since each septum 

 was at one time the end of the body-chamber, similar marks are to be found on the 

 concave side of each, but they do not correspond to any arteries in the septal 

 chambers, which are not in communication with the siphuncle in this way. 1 Similar 

 vascular marks are found on the internal casts of chambers broken off from Silurian 

 Cephalopods, e.g. 0. imbricatum. 



(6.) Bands on the septa. — In some Silurian forms a raised band in the cast, corre- 

 sponding to a depression on the concave side of the septum, passes from the siphuncle 

 to the circumference, usually but not always in the line of symmetry. Such are 

 figured on Plate XIY. as belonging to D. imbricatum. Nothing corresponding to 

 these is to be seen in the recent Nautilus, and they are very rare in the Orthocerata, 

 and one can only suggest that they may have been special lines of attachment for 

 the mantle. 



(c.) Normal lines. — This name has been given by D. D. Sandberger 2 to narrow 

 lines which are occasionally seen in the cast running longitudinally, and usually in 

 the plane of symmetry. In some cases they appear raised, in others depressed. One 

 of these can be found in the recent Nautilus, on the concavity of the ventral side 

 of the body-chamber, before the septal deposit conceals it, but it is very obscure and 

 narrow. On the dorsal side there is, as already noticed, in connection with the 

 septa, a shallow furrow leading from septum to septum and deepening anteriorly, 

 and this in a cast would produce a well-marked normal line. In some fossil Ortho- 

 cerata, e.g. 0. semipartitum, this is so prominent, and the deepening at each septum 

 so marked, as to have been taken for the siphuncle. Its real significance, except its 

 connection with a longitudinal line noticed on the surface of the mantle, is still 

 obscure. Occasionally such normal lines or keels are more numerous, and divide 

 the whole surface into equal parts. 



(d.) Folds at the base of the body-chamber. — The surface near the base of the body- 

 chamber, and on the adjacent septa, is sometimes thrown into longitudinal folds, the 



1 And they cannot therefore be adduced as arguments for the vitalising function of the siphuncle. 

 See Prof. Owen, loc. cit, p. 971. 



2 ' Versteinerungen von Nassau.' 



