80 CEPHALOPODA. 



17. Coronati . . Oolitic. — Ex. A. Humpliresianns. 



18. Macrocephali . Oolitic and Cretaceous. — Ex. A. microstoma. 



19. Globosi . . . Trias. — Ex. A. globus. 



20. Fimbriati . . Jurassic and Cretaceous. — Ex. A. subfiuibriatus. 



The more modern and more generally accepted subdivi- 

 sions of Ammonites are founded principally upon differences 

 in the form of the body-chamber, the shape of the aperture 

 of the shell, and the presence or absence of the structure 

 which will be immediately described as the Ajityclms, to- 

 gether with the characters of this organ when present. As 

 regards the first of these features, the length of the body- 

 chamber is very variable in different forms of Ammonites, 

 and this variability must have been correlated with corre- 

 sponding important differences in the organisation of the 

 animal itself. In a great many forms, especially those from 

 the Jurassic and Cretaceous formations, the body-chamber is 

 comparatively long, varying from half a whorl to two-thirds 

 of a w^horl, or reachino; a whole wdiorl or more in length. 

 In other forms, however, the chamber of habitation is very 

 short. 



The shape of the aperture of the shell is likewise very 

 variable. In many forms there is a constriction of the shell 

 just behind the aperture; and the transverse sulci, wdth inter- 

 vening ribs, which cross the shell of many species, are simply 

 remains of the constrictions marldng the position of the aper- 

 ture at former periods of growth. In other cases, the aper- 

 ture of the shell is furnished with larger or smaller prolonga- 

 tions or processes, which may be situated on the sides of the 

 mouth, or on its upper aspect, or, rarely, below. When a 

 shelly process exists upon the upper side of the aperture, it 

 probably serves, as supposed by Mr J. F. Blake, to jDrotect 

 the funnel ; but the object of the much commoner lateral 

 processes (as seen in Ammonites Jason, fig. 466), and of the 

 unusual lower extensions of the shell, must be regarded as 

 quite uncertain. 



Lastly, we- have briefly to consider the nature and prob- 

 able function of the extraordinary organ known as the 

 Aptyckiis. The structure so called (fig. 470) is a calcareous 

 or horny plate, which has commonly been found just outside 



