ANIMALS INHABITING MULTILOCULAR SHELLS. 275 
matter of some difficulty, even in large specimens, to discover where- 
abouts the aperture for the siphuncle is situated. 
Now, with regard to the use of this extreme complication of the 
edge of the chamber, although it certainly seems to add much 
strength to the whole, there is a secondary object mentioned by M. 
von Buch, which we must not neglect to notice. He considers that 
the alternate projections and recesses thus formed must have given 
firmer hold to the mantle of the animal, and enabled it to retain, in 
safe and close connexion, the animal and its shell, notwithstanding 
the small size and inconvenient position of the siphuncle, which, in 
all probability, is of much use for this purpose in the Nautilacea, 
Following out the idea, he has traced a remarkable uniformity in 
the number and positions of the undulations throughout the whole 
genus of Ammonites. ‘I think,” he says, “it may be considered a 
question definitively settled that in all species, whatever may be the 
apparent anomalies of form, it is easy to make out six principal 
lobes,* with other accessory lobes interposed, which all adjust them- 
selves with wonderful regularity in the circumference of the shell.” 
Between every pair of depressions or lobes there is a raised rounded 
part, the saddle ; and these saddles always correspond in number 
and position to the lobes. 
The system of lobes and saddles, so constant and disposed with 
such exact symmetry, appeared to M. von Buch to indicate an orga- 
nization separating very decisively the two families Nautilacea 
and Ammoneata ; and although, perhaps, the conclusions drawn . 
from this view of the subject, and the sub-divisions proposed, are 
premature, and the facts already determined not quite so important 
as they have been assumed to be, still much good has been done by 
the mere convenient classification of so large a genus into distinct 
groups, which, indeed, our author goes so far as to call “ une distri- 
bution claire, positive et utile, en familles naturelles.” 
We have one more remark to make on the Ammonites generally, 
before proceeding to the consideration of the allied genera. Their 
shells, it would seem, are usually thin, and the siphuncle small ; but 
the former receive great additional strength by the number of ribs, 
as well as tubercles, not immediately connected with the septa, and 
fluting, as it were, the part most exposed to pressure; while the 
* By the word “lobe” is designated the depressions which occur in the 
wall of the chamber of Ammonites, &c. caused by the successive bends of the 
undulations towards the aperture. The raised parts between these hollows 
are called saddles. 
