KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 23. N:o l|2. 27 
surface in some places a peculiar appearance as if having facets (fig. 1, 15). The inter- 
nal cast (Pl. II fig. 8 a) of the neck shows very fine, interrupted, irrregular, transversally 
impressed lines, probably produced by the mantle of the animal. The septum forming the apex 
is of the same shape as those in the Nautiloid, sometimes much convex ( fig. 11). On the inner 
side it is much strengthened through depositions, so as to be almost as thick as the shell. 
There is no intermediate septum between this and the sigmoid ones. As the numerous 
large and well preserved specimens of this species are instructive for the morphology of 
the sigmoid septa, these may here be described in detail. They are generally seven, 
though there are specimens which have only six septa, and again there is one (PI. II 
fig. 18) where there has been forming an eighth septum of the common concave or watch- 
glasslike shape on the bottom, above the sigmoid septa, being a remarkable recurrence 
to the initial stage. Again there are specimens, as for inst. that in connexion with the 
Nautiloid (Pl. III fig. 17), without the least trace of any septum. Such shells must be 
regarded as so recently formed or young that no septa had yet been secreted in them 
(Aphragmites), and we learn by this as well as from other species, that the shell of the 
Åscoceras stage was, as to its exterior, quite ready formed or at least nearly so before 
any septa were secreted. Such specimens show how little real value there is in such a 
genus as Aphragmites. Quite different it is when there are adult specimens of Ascoc. 
manubrium without any septa at all, these having been destroyed in the interior, but have 
left a trace of their former presence by their outlines on the surface of the nucleus. 
The first sigmoid septum is quite entire and complete. But already the next, the 
second one, has a large central lacuna which is hemmed in on all sides by a narrow frame, 
at the points, where this septum is in contact with the first septum (See Pl. ITT fig. 8). The 
third septum in the same way embraces the second septum and its lacuna is consequently still 
larger. Ås a constant rule the higher the septum is situated or the longer it is, the larger is 
the lacuna. Thus, in this species, the seventh has the largest lacuna. The third septum is 
largest of all (fig. 18 c pl. IT) in so far it is developed. Its arched superior portion is 
nearly twice as large as the corresponding one of the first septum. This arch of the 
septa is, as seen on pl. II fig. 17, provided with a shallow sinus at its inferior margin. 
If we compare PI. II fig. 16 and fig. 18 of two specimens of almost equal size, the latter 
sectioned vertically along its median axis, there will be remarked the great difference in 
the size and appearance of the septa. The former representing the nucleus, after the shell 
has been peeled off, has all the septa in juxtaposition, showing their exterior outlines or su- 
tures where they have been secreted in a narrow strip above each other and where they 
thus far are perfect. Now it is to be borne in mind, that the septa do not continue across 
the shell in a straight line, so as to be as large in the interior as seen on the outside. 
Their surface in the interior is bent in a semicircle (see pl. II fig. 21) so that in a transverse 
section they are seen to be widest near the lateral surfaces of the shell and most narrow 
in the diameter between the dorsal and the ventral sides. In the section represented in Pl. 
II fig. 18 again they have been cut where their innermost curve is situated. A model, construc- 
ted according to several specimens, and figured in pl. III fig. 8, elucidates these peculiarities. 
On the ventral side the septa are quite regular, although there is very little place 
left for them to develop through the large size of the siphuncle. When seen from above 
