﻿BEITISH FOSSIL CEPHALOPODA. 37 



siphuncle. When the ornaments slope in one direction and the septa in the 

 opposite, as in 0. Grayi, the two influences balance each other, and we find a 

 central siphuncle. These connections are sufficiently constant to suggest some real 

 relation, though in several cases it may be disturbed by other influences. 



The position of the siphuncle is not always constant during the growth of 

 single individuals. In some, beginning eccentrically, it ends by being central ; but 

 more often it is central at first, and diverges from the middle line with age. In 

 species which admit of this change, it may take place sometimes earlier than usual, 

 so as to give the idea of a different position throughout. These variations do not 

 nullify the value of the character in the determination of species, but only suggest 

 care in its observation. Another very rare anomaly is the unsymmetrical position. 

 One is tempted to believe that this is due to contortion, but it is found in specimens 

 showing no signs of such treatment. In some cases, perhaps, it may be due to 

 disease, as possibly the Nautilus anomalus of Barrande may be the same as N. vetustus. 

 There are, however, instances, e.g. 0. intermedium, in which it is unsymmetrical in 

 all specimens, and then it must be regarded as a specific character. 



It is to be noted that, in determining the position of the siphuncle when the 

 septa are oblique, it must not be measured on the septal but on the transverse direct 

 diameter of the shell, since a point truly central will lie on the side nearer the apex 

 in an oblique section of a cone. 



(6.) Its size. — The variations in this are obviously correlated to those of its 

 position, because when it finally settles in the Ammonitoidea at the external border 

 it becomes, and ever remains, filiform in size. Most usually it is small ; but it 

 becomes large in two groups from different causes. In one, Endoceras, it has a 

 large diameter throughout; in the other, Actinoceras, it is swollen between the 

 chambers. The size is not always constant throughout the shell in proportion to 

 the diameter ; even in the recent Nautilus it is proportionally much larger in the 

 young, and this seems to be the usual variation ; in fact, it tends to remain of the 

 same absolute size throughout. The remarkable siphuncles of 0. docens, Barrande, 

 and that figured by Dr. Bigsby in the Transactions of the Geological Society, 

 2nd series, vol. i. PI. 30, even show a rapid decrease in the absolute size ; but 

 I have not seen any such from British strata. In Ascoceras, also, the same is seen ; 

 the siphuncle of the last regular septum is large, but that of the distorted ones gets 

 pushed to one side, and its size rapidly decreases. 1 



(c.) Its structure. — The actual membranous siphuncular tube itself is a soft 

 organ, of which no trace remains in the dried shells. These only show the two 

 exterior coverings of it. In Nautilus pompilius the inner of these is a black or 

 dark brown horny tube, which is not strictly continuous. The base of each neck of 

 the septa is swollen internally, and to the hinder part of this swelling is attached 

 1 See Barrande, ' Syst. Silur. de Boheme,' vol. ii. PL 513. 



