972 PROF. OWEN ON THE [Nov. 19, 



rnollusks suggests to the physiological mind the probability that 

 some corresponding step in advance of the ordinary condition of a 

 testaceous defence would be likely ; and the retention of the parts of 

 such defence vacated in the course of growth, with the superaddi- 

 tion of a vascular by-way running through the whole, verifies such 

 anticipation of the means whereby the camerated and siphoniferous 

 shell is thus brought into closer harmony with the rest of the organic 

 structure. 



When the first simple, single-chambered, nuclear dwelling is added 

 to and, in part, abandoned, a closer connexion is therewith preserved 

 than in Vermetus, e. g. ; and, as the artery penetrating the mem- 

 branous siphon continued from the last-formed dwelling-chamber 

 was demonstrated by mercurial injection 1 , there is no reason to 

 doubt that such organic connexion was maintained between the 

 fabricator of the second chamber after it had advanced from and 

 vacated the first or nuclear one. 



The constancy of this siphuncular connexion running through all 

 the chambers of the largest and most complex of the polythalamous- 

 shells, with the great size and singular complexity of the siphuncle 

 in several extinct species, form the grounds on which I still bold to- 

 my original belief in the function of the siphuncle as related to a 

 maintenance of the vitality of the shell. But this relation may 

 \>e connected, also, with a greater share assigned to the siphon in the 

 protection of the soft parts of the Cephalopod at the earlier stages of 

 its existence. 



The chief character of the tetrabranchiate chambered and sipho- 

 nated shell is its affording, besides a sheath or case to the whole 

 animal, a special protection to a part of the animal. 



Such twofold office is performed by the shell in certain Gastro- 

 pods, conspicuously in the genus Calyptreea, in which an accessory 

 "cup," springing from the concavity of the larger " saucer," lodges 

 part of the muscular system 2 . 



Every chambered and siphonated shell begins in this simple fashion. 

 The protoconch is cup-shaped or flask-shaped, and includes a similar 

 but smaller blind beginning of the siphon. 



The proportion of the inner partially protecting shell to the outer 

 wholly encasing shell is greatest in the Silurian Orthoceratites ; and 

 with the large proportional siphon are associated complexities cha- 

 racteristic of the genera Ormoceras, Huronia, &c. 3 



Modifications of the contents of such siphons in the Silurian Vagi- 



1 "The lesser aorta" sends off a small branch (14, pi. 5 & 6) "which, winding 

 round to the ventral aspect of the ventricle (to which it is connected by a pro- 

 cess of rnenibrame), passes through a foramen in the septum dividing the peri- 

 cardium from the cavity at the bottom of the pallia! sac, is then continued 

 through that cavity, passing between the ovary and gizzard, and lastly enters, 

 without diminution of size, the membranous tube that traverses the parti- 

 tions of the shell." — Memoir on the Pearly Nautilus, p. 36. 



2 "Its cavity is filled by what maybe termed the apex of the foot, which here 

 loses its muscular character, and assumes a gelatinous texture."—" Anatomy of 

 the Calyptraidar," Trans. Zool. Soc. vol. i. p. 208, pi. 30. figs. 2, 6/. *8*r: JX35~. 



3 Charles Stokes, in Geol. Trans. 2nd scr. vol. v. p. 706, 1837. 



