KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 23. N:o |2. 15 
this continuity is broken (See pl. III f. 9—11). There it is seen that the first sigmoid 
septum alone is entire, and all the following are open or lacunose along their centre, the 
organic deposit has not been secreted there (See pl. III f. 8, 10—11). The deposition 
has ceased where a septum has touched the surface of the next preceeding septum, but 
around the margins, where they do not meet, all are entire. The margins thus form a 
sort of a frame around a central, empty space and there is a sort of imbricate arrangement 
of them in their position relatively to each other. In a certain way the curvature of the 
septa may, as HraTtTtT has remarked, be compared with the saddle of the septa in Gonia- 
tites, though much more exaggerated in Ascoceras. Moreover, on the interior of the 
dorsal side in the shell the septa form a semicircular sinus so that they are widest along 
the sides and most restricted in the middle. The siphuncele, always near the ventral side, 
consists of one element less than the number of the septa, and the elements are almost 
always broad, nummuloid, rapidly increasing in breadth upwards. This siphuncle stands 
in immediate connection with the siphuncle of the Nautiloid through a little peculiar tu- 
bular duct, which is different in each species and is closed with a calcareous secretion, 
when the decollation has occurred. The earlier septa are more distantiated from each 
other, the uppermost being closer together, the distance between the first and second, and 
between the second and third being the largest. Interiorly, in the centre of the shell, the 
2d, 3d, 4th advance furthest and the uppermost again are most narrow and receding. In 
Glossoceras again there is a steady increase in breadth and the uppermost are most pro- 
minent of all. Along the dorsal side the septa are bent in an inwards directed curve, 
then after a more or less broad swing they again closely approach the dorsal side and 
lastly turn across the shell towards the ventral side which they encircle. 
At the first sight of the almost sudden change from the Nautiloid to the Ascoceras, 
it might readily be supposed that most important modifications in the nature of the 
mollusc, in its structure and functions must have stepped in. And no doubt, there has 
been some change, at least in volume, as it seems to be evident that the shell has been, 
as it were, moulded on the body of the animal. But when we again find, that there in 
several instances is evidence of a curious reversion in the shape of the septa and siphuncle 
to the Nautiloid stage in the Ascoceras, after all the sigmoid septa have been there for- 
med, it is questionable whether there really has been such a great change in the shape 
of the animal itself, as the altered form of the septa seem to imply. It would namely be 
very strange if the animal could thus twice modify its body, so as to revert again to a 
Nautiloid shape, after having once changed it. Of this curious reversion of characters in- 
stances may be seen in the specimens delineated pl. I fig. 21—24, pl. II f. 18, pl. IV 
f. 34, showing a specimen with three nautiloid septa, pl. V f. 22, 22 a, with four regu- 
lar septa above the sigmoid ones. Further on these forms are described more in detail. 
There are specimens (Pl. V f. 26—27) which show, that the Ascoceras shell as a 
direct continuation of the Nautiloid was completed without having a single septum interiorly. 
It cannot be doubted that this shell has been an exterior one or enclosed the animal and 
not vice versa. The rich ornamentation of its surface, the long body chamber in the 
Nautiloid and the deposit of fresh calcareous strata from within, after the decollation, testify 
this sufficiently. In the shell of Spirula enclosed within the body of the animal there is 
