15 
of Spirula australis, Lamarclc. 
fundus of the last chamber * *, only with reversed proportions ; 
such visceral mass occupies a smaller proportion of the last 
chamber in Nautilus , yet consists of a larger proportion of the 
viscera than in Spirula. Although by the forward extension of 
the last chamber beyond the muscular attachments, in Nautilus , 
a still greater proportion of the animal is contained, or may be 
retracted, within that chamber—nevertheless, in the degree in 
which the mantle may be reflected over the exterior of the 
shell, such proportion of the shell may be regarded as internal. 
Lateral prolongations of the mantle can extend over the umbi¬ 
licus, and are the efficients of the deposition there of coloured 
calcareous matter, in Nautilus pompilius j and such pallia! pro¬ 
longations are homologous with the lateral terminal lobes of the 
mantle of Spirula (PI. I. figs. 1, 2,3, c, c), which similarly cover 
the umbilical parts of its shell. 
The distinction, therefore, between Nautilus and Spirula , 
in regard to the shell, in its protective relation, is relative, not 
absolute : in the one a small proportion of the shell is occa¬ 
sionally “ internal; ” in the other a small proportion is always 
u external: ” in both the multilocular shell corresponds with 
the phragmocone of the Belemnitet. 
The tetrabrancliiate Orthoccras may be called a representa¬ 
tive analogue of the dibranchiate Belemnite, as the tetra- 
branchiate Ammonite is of the dibranchiate Spirula. The 
siphon is u ventral ” and u marginal ” in both kinds of coiled 
shells, but it runs along opposite sides of the coil. In Spirula, 
its position is 11 internal ” or 11 entomarginal; ” in Ammonites 
it is 11 external ” or “ ectomarginal.” 
EXPLANATION OF THE PLATES. 
Plate I. 
Fig. 1. Right-side view of Spirula australis , in outline, showing the four 
arms (l, 2, 3 , 4 ) and the tentacle (d) of that side, a, eye-aper¬ 
ture; b, dorsal pointed lobe of fore border of mantle ; c, umbilical 
fold of mantle; e, exposed part of outer wall of shell on the ven¬ 
tral side; /, the same on the dorsal side ; g, longitudinal marks 
sorte de calotte se continuant par sa circonference avec l’enveloppe de 
celle-ci, et donnaut, au fond et vers le bord inferieur, naissance a un 
prolongement tubiforme penetrant et s’attachant dans le siphon de la 
premiere cloison, puis se continuant sans autre adherence, a ce que je 
suppose, jusqu’a son origine vers le sommet de la coquille, dont j’ai pu, 
en effet, ie retirer sans briser, dans une longueur considerable; en sorte 
que l’on peut dire que celle-ci est dans un prolongement du muscle colu- 
mellaire ou retracteur de la tete et de ses appendices, et que le siphon 
membraneux n’est lui-meme qu’une partie de ce muscle/’ — Op.cit. p. 379. 
* Op. cit. pi. i. a. 
t See the restoration of this extinct form in my ‘ Lectures on the 
Comparative Anatomy and Physiology of the Invertebrate Animals,’ 
8vo, 1843, p. 333, fig. 133, b. 
