CEPHALOPOD BEGINNINGS. 



433 



Fig. 5 



II a 



Longitudinal 



possessed 'sheaths' Hke those of Piloceras and some species of Endo- 

 cevas. The faint streak described by Clarke is nothing more than one 

 may see any day in a common belemnite guard, and merely indicates 

 a shght pause in the deposition of the calcite, of no morphological 

 significance whatever. The sheaths of Piloceras, well shown in 

 Fig. 5, I, are clearly defined structures of a totally different origin. 

 It may be worth while considering what 

 that origin is. It seems pretty clear that 

 in most of these archaic Cephalopoda 

 the visceral cone extended to the depth 

 of at least two loculi down the siphun- 

 cular passage. Since, like the visceral 

 hump, it had the power of secreting 

 shell, it formed, when in this position, 

 long septal necks which often fused to 

 form the neck-tube. We know that in 

 Nautihis and Spinila, after the secretion 

 of the septal necks, the outer coat of 

 the siphuncle, both inside and outside 

 the region of the septal neck, becomes 

 hardened by calcium carbonate ; this 

 gives it a certain rigidity, and assists 

 its retention in the fossil state. The 

 same thing must have occurred in the 

 coat of the visceral cone. Now, in 

 Piloceras, when the animal advanced in 

 the shell its viscera naturally followed 

 it, and by suction the walls of the 

 visceral cone were drawn in so as to 

 form the narrow and empty siphuncle. 

 At least such would have been the 

 case had not the stiffness of the outer coat 

 prevented complete yielding of the skin, 

 especially at the posterior part where 

 the siphuncle tended to begin, but where 

 the coat was most calcified. It must 

 therefore have happened that the inner 

 layers of the skin were gradually torn 

 away from the outer layers. Another 

 stiffening of the skin would take 

 place higher up, and the process would be repeated. Thus the 

 ' sheaths ' of Piloceras are nothing more than the remains of these old 

 coats, left hanging on the siphuncle all the way up the passage (Fig. 5, 

 I, V and s). The siphuncle is of course the structure that is often per- 

 plexingly called the " endosiphon." In some species of Endoceras the 

 secondary calcification of the skin of the visceral cone seems to have 

 advanced further, and the siphuncular cavity is divided into chambers 



2 F 



Piloceras. 

 secdon of the siphuncular pas- 

 sage; 9. quartz filling the greater 

 part of the neck-tube cavity ; 

 »i, matrix occupying the former 

 position of the visceral hump ; 

 s, ' sheaths,' or calcified coats of 

 the visceral hump ; e, the 

 siphuncle or so-called ' endo- 

 siphon ' ; r, remains of former 

 sheaths (?). II. Posterior ex- 

 tremity of a similar specimen, 

 much eroded, but showing an 

 aperture, a, at the apex ; v>. 

 the neck-tube; q, quartz in- 

 filling between neck-tube and 

 one of the sheaths. III. Trans- 

 verse section of a similar speci- 

 men, showing the outer wall or 

 neck-tube and the siphuncle e ; 

 / is a partition the appearance 

 of which is exaggerated and the 

 significance of which is unknown. 



(All figures of natural size ; drawn 

 by A. H. Foord; the block kindly 

 lent by Dr. H. Woodward, F.R.S.) 



