MOLLUSCA — CEPHALOPODA. 



153 



The same models show the change from septal necks to the Gallery 

 collars characteristic of later Ammonoidea. TalWe-case 



Among other specimens shown in this Case are those i. 

 illustrating the lid or operculum of the ammonite to which, 

 when it was supposed to be an independent shell, the 

 name A;ptychus was applied (Fig. 83). 

 Some of these specimens preserve the 

 aptychus in its natural position com- 

 pletely closing the opening of the shell. 

 The aptychus consists of two equal 

 halves (sometimes not divided), and there 

 is no good reason for doubting that these 

 were formed as calcifications of that 

 structure which in a nautilus is called _^ ^ ^-^^^^ 



the hood (see the preparation of Nautilus 'an ammonite, 

 between Wall-cases 13 and 14). 



Eeturning to the specimens and models that illustrate Table-case 

 the structure of the Coleoidea or Belemnoidea, we study ^' 

 first a belemnite from the Lower Lias of Charm outh that was 

 described by Huxley in 1864 (Memoirs of the Geological 

 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 

 front of the mantle-folds, and on each side of it is an eye. 

 Behind this is the body- chamber, protected on the back by 

 the pro-ostracum, not seen in this view. The viscera are 

 not shown, since nothing definite is known about them, with 

 the exception of a small bag, 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 



