SUPPLE:vrEXT. 
3S9 
from the ventral to the dorsal side, as may he seen in many casts, 
where the sutures continue uninterruptedly across the shell. But in 
the interior of the shell this continuity is broken (fig. 84, B, C, D). 
There it is seen that the first sigmoid septum alone is entire, and all 
the following are open or lacuuose along their centre ; the organic 
deposit has not been secreted there (fig. 84, A-D). The deposition 
ceased where a septum has touched the surface of a preceding- 
septum, but around the margins, where they do not meet, all are 
entire. The margins thus form a sort of 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 curva- 
ture of the septa may, as Hyatt has remarked, be compared with the 
saddle of the septa in Goniatites, 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 siphuncle, always near 
the ventral side, consists of one element less than the number of the 
septa, and the elements are almost always broad, nummuloid, and 
rapidly increasing in breadth upwards. This siphuncle is in im- 
mediate connection with the siphuncle of tho Hautiloid part of the 
shell through a peculiar little tubular duct (fig. 84, A, dt), which 
is different in each species, and is closed with a calcareous secretion 
when decollation has taken place. The earlier septa are more dis- 
tant from each other than the later or upper ones, the distance 
between the first and second, and between the second and third being 
the greatest. In the central part of the shell, the second, third, and 
fourth septa advance furthest, and the uppermost are the narrowest 
and most receding (fig. 84, A). In Glossoceras, again, there is a 
steady increase in breadth, and the uppermost septa are the most 
prominent of all. The form and position of the septa (fig. 84, A) 
may be thus described : commencing at the dorsal wall of the shell 
they first make a strong, inwardly-directed curve, and then sweeping 
outwards in a wider curve, they closely approach the dorsal side and 
finally bend round to the ventral side, thus completely encircling the 
shell. 
Dr. Lindstrom questions whether there was any important change 
in the animal of Ascoceras, such as the altered form of the septa in 
the Ascoceras stage would appear to indicate. That there was some 
change, at least in volume, may be reasonably supposed, because it 
is evident that the form of the shell must have been governed, to a 
great extent, by that of the animal’s body. But in several cases 
there is evidence, in the shape of the septa and siphuncle of tho 
