﻿BEITISH FOSSIL CEPHALOPODA. 33 



far the enormous lobe of the Tretoceras found in this position and reaching back more 

 than one chamber is an extraordinary development of this small dorsal prominence. 



The second projection is that known as the neck of the siphuncle. It is usually 

 open at the posterior end, and so has the shape of a funnel. In the first septum, 

 however, of Nautilus pompilius it is closed (PI. II., fig. 4, e). The projection is com- 

 paratively slight, on account of the shallowness of the nucleus, and its base rests 

 against the interior of the shell opposite the end of the cicatrix. The siphuncle is 

 thus provided with a point oTappui, and in the motion forward of the animal to form 

 the next septum it is enabled to pierce the mantle, so that the second neck is open- 

 It is, however, very long (fig. 4, / ) on the convex side, where it reaches very nearly 

 across the space between the chambers, 1 and its substance is thrown into irregular 

 folds as of a membrane enclosing too small a tube. The third neck is shorter and is 

 also thrown into folds, and so they gradually assume their normal length. In all 

 the true Nautiloids these necks are directed backwards, like the general convexity of 

 the septa ; whereas the reverse is the case in the Ammonitoidea, as was first pointed 

 out by Yon Buch. 2 The essential relation between the general direction of the con- 

 vexity and that of the neck is shown by the fact that in Dibranchiates such as 

 Spirula, in which the septa are again concave to the aperture, the neck is situated 

 like that of the early chambers of the Nautilus. This, therefore, is not an inde- 

 pendent distinction between the suborders. 



The early students of the Orthocerata* laid great stress on the length of the neck ; 

 distinguishing as separate groups those in which it reaches back to the next septum, 

 and so providing a complete sheath to the siphuncle, from those in which it is short. 

 In this genus it appears that other characters, as the largeness of the siphuncle, are 

 associated with this form of neck, and hence the subgenus Endoceras; but the 

 sheathed siphuncles are met with in other genera, and in fossils it is often exceed- 

 ingly difficult to say which character the neck had, for both it and the covering of 

 the siphuncle have been so crystallised that their distinctness from each other is lost. 



It is a common feature to find these necks 4 ending in an elegant curve, turning 

 away from the siphuncle and forming in some the basis of a mineral deposit. 



(<i.) Mineral deposits in them. — The septa themselves are very narrow. The error 

 into which Stokes and M'Coy, among others, fell, of mistaking the crystalline 

 coverings acquired by fossilisation for the septa, has long been exploded. There are 

 found, however, in certain specimens, a different kind of deposit on the septa to the 



1 Hyatt, loc. cit., states that this second neck is .not perforated, but is continued round the 

 siphuncle between it and the first neck. My observations show that this is not the case. The figure 

 given by Hyatt shows it was not observed to do so in his specimen, and from the comparative short- 

 ness of the third neck it is very improbable that it should. 



2 Von Buch, ' Ueber Ammoniten.' 



3 Munster, ' Beitrage zur Petrefactenkunde ;' and Quenstedt, ' Petrefactenkunde Deutschlands.' 



4 It may not be amiss to state that the French call this a " goulot," and the Germans " Diite." 



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