MOLLUSCA — C]-:rHAL0r01)A. 
149 
this mode of growth (Fig. 80 b). The shrinkage appears to 
have been greater from the first, and thus the septa stretch 
across the conical shell, dividing it into a series of chambers, 
and leaving only a narrow neck -tube in which there are no 
sheaths. The part of the shell-cavity in front of the last- 
formed septum is called the body-chamber, and in it was the 
main mass of the animal. From the visceral hump, how- 
ever, proceeded the fleshy siphuncle, passing through each 
septum to the ape.v of the shell. The line along which the 
septum is attached to the shell-wall is called a suture, and in 
these simple forms passes regularly all roiind the shell. 
There are numbers of straight shells of this simple type, 
but those that are the most completely known may be divided 
into two groups by the presence or absence of a small, more 
or less globular, initial chamber. This, which is generally 
separated from the next chamber by a slight constriction, is 
called the protoconch (first shell) and believed to be the shell 
of the embryonic cephalopod. It is well seen in some specimens 
of Bactrites (Fig. 81 1c). Often this protoconch seems to have 
been lost in the adult, and in its place is seen only a scar or 
cicatrix denoting its former presence (Fig 81, c, c, m). 
At an early period in the history of the cephalopod race 
the shell began to curve, and this curvature increased until 
the shell was coiled on itself. Such a coiled shell was far 
more manageable than the long shell of an Orthoceras, and 
was less liable to damage. And so it is found that the long 
straight shells gradually die out and give place to coiled 
shells. Now, just as there were two types of straight shells, 
so were there two of coiled shells : one with a protoconch, as 
may be seen in models of early goniatites (Fig. 81 n) ; the 
other without a protoconch, as shown by the model of 
Nautilus (Fig. 81 a-c). Further examination of the shells 
of these two types reveals other differences. The early 
coiled shells with a protocouch are long, narrow, smooth, 
with septa usually far apart, and with a long deep body- 
chamber (Fig. 81 n). Those without a protoconch are short, 
broad, often with a longitudinal ornament, with septa 
relatively close together, and with a shallow body-chamber 
(Fig. 81 a). In later forms of these two types other 
differences appear, such as will be realised by comparing 
an ammonite (which is one of the former series) with a 
Nautilus (lig. 82). Generally speaking the siphuncle of an 
ammonite is close to the outside of the coiled shell; the 
edges of the septa are folded, so that the sutures are coinj)!!- 
Gallery 
VII. 
Table-case 
1 . 
