MOLLUSCA — CEPH.VLOPODA. 
153 
Fig. 83. — Aptychus of 
an ammonite. 
I’he same models show the change from septal necks to the 
collars characteristic of later Ammonoidea. 
Among other specimens shown in this Case are those 
illustrating the lid or operculum of the ammonite to which, 
when it was supposed to be an independent shell, tlie 
name Aptyclms was applied (Fig. 83). 
Some of these specimens preserve the 
aptychus in its natural position com- 
pletely closing the opening of the shell. 
Tlie aptychus consists of two equal 
lialves (sometimes not divided), and there 
is no good reason for doubting that these 
were formed as calcifications of tliat 
structure wdiich in a nautilus is called 
the hood (see the preparation of Ncmtilns 
between Wall-cases 13 and 14). 
Eeturning to the specimens and models that illustrate 
the structure of tlie Coleoidea or Belemnoidea, we study 
first a belemuite from the Lower Lias of Charmouth that was 
de.scribed by Hu.xley in 1864 (Memoirs of the (leological 
Survey). This shows that the guard is small as compared 
with the size of the whole animal, and that only a small part 
of the shell-cone is contained within the guard (compare 
Figs. 84 and 85 a). The wall of the body-chamber extends a 
considerable distance in front of the chambered portion of 
the shell (or phragmocone) and is known as the pro-ostracum. 
In front of this is the head of the animal, in which may be 
seen the beaks; and in front of this again are six arms 
bearing hooks. Other bookless arms may or may not have 
been present. Upon this and other specimens is based the 
exhibited diagram of a belemnite animal. This is seen from 
the under side, with a part of the skin removed from the 
middle region and the shell sliced down the middle. In 
addition to the short hooked arms seen in the fossil just 
mentioned, are shown two long arms, of which, however, 
traces have not been found. The funnel is seen emerging in 
Iront of tlie mantle-folds, and on each side of it is an eye. 
Behind this is the body- chamber, protected on the back by 
the jn-o-ostracum, not seen in this view. The viscera are 
not shown, since nothing definite is known about them, witli 
the exce[)tion ot a small bcag, from which a tube leads 
forward and opens into the funnel. This bag is filled with a 
carbonaceous .substance which, as known in living cephalopods, 
can be ejected in the form of a dark ink. It serves to 
Gallery 
VII. 
Table-case 
1 . 
Table-case 
1 . 
