113 
The shells of this genus, Plate VII. Fig. 14, like the nautili, are 
divided into chambers by septa, through which passes a tube or 
siphunculus, varying much in its form, situation, and size, in dif- 
ferent specimens. 
The surfaces of these fossils vary so considerably, as to form very 
striking differences, on which may be grounded their separation into 
different species. In some their surface is smooth, and in others 
marked by longitudinal or transverse striae. The parts possessed by 
the chambers are in some marked, externally, by a considerable de- 
pression ; whilst in others, these corresponding parts are distinguished 
by a slight degree of protuberance. They differ, also, with respect 
to their form; some being straight, while others are described as 
slightly curved at their smaller end. 
Their size varies, from being, as has been just observed, so small 
as to be a microscopic object, as to equal the thickness of a man’s arm. 
M. Walch observes, that they are sometimes nearly four inches in dia- 
meter, and more than an ell long, possessing nearly seventy chambers. 
A specimen now before me is full four inches in diameter. Dr. Wright 
describes an orthoceratite in marble, two feet four inches in length, 
in which were sixty-six partitions. Phil. Trans. Vol. xlix. p. 670. 
Some care is however necessary, before we determine on the actual 
form of such of these fossils, as, being imbedded, have had their parts 
displayed by section. If we suppose a spirulite thus imbedded, and a 
section commenced at its straight part, should the section not happen 
to be made on the same plane with that on which the spiral part is 
disposed, a part only of the last turn may be cut through, by which 
the straight part may be separated from the spiral, and the part of the 
last turn left with the straight part, might lead those who did not 
take this circumstance into consideration, to consider it as a bent 
orthoceratites. Indeed, so easy it is for mistakes to have been thus 
made, that I should be disposed to look with suspicion at all the sec- 
tions of orthoceratites bearing this bent figure, were it not that ana- 
VOL. III. o 
