CEPHALOPODA. 



333 



years been in the main sufficient, in my opinion, to determine its affi- 

 nities: but as the value of this evidence could only be appreciated by 

 those who had studied the laws of correlation of the cephalopodic 

 structures, it failed to produce general conviction ; and the additional 

 facts which I am able to-day to bring before you will not, therefore, be 

 unacceptable. The shell to which I allude is that called the ' Be- 

 lemnite,' which is associated with the more obvious congeners of the 

 Nautilus through a considerable range of the secondary rocks. 



The chambered part of the shell of this extinct Cephalopod has the 

 form of a straight cone {fig- 133. h), the septa being numerous, with a 

 slightand equable concavity directed towards 

 the outlet or base of the cone. The inter- 

 vening chambers are so shallow that the septa 

 have been compared to a pile of watch-glasses. 

 The septa are chiefly composed of nacreous 

 substance, with a thin layer of opake and 

 friable calcareous matter on both surfaces, 

 and the entire cone is enveloped in a slieath 

 of opake calcareous matter lined with na- 

 creous substance, and having on the outer 

 surface, or there degenerating into, a pellicle 

 or thin layer of a horny substance. This latter 

 horny or albuminous tissue is continued 

 forwards beyond the base of the chambered 

 cone*, and forms the parietes of a cavity («) 

 containing some of the viscera of the animal. 

 The chambered cone itself, with its 

 sheath, is lodged in a conical cavity or 

 alveolus -|- excavated in the base of a long 

 conical or fusiform, sometimes compressed, 

 spathose body (c), resembling the head of a 

 dart or javelin, whence the name ' Belem- 

 nite,' applied to this genus of extinct 

 Cephalopods. This sfjathose sheath or 

 guard of the chambered cone is most com- 

 monly found detached from the rest of the 

 shell, with its thin aveolar portion fractured, especially when fu- 

 siform and in the younger Belemnites, when it is pointed at both 

 ends. In this state it has been mistaken for the spine of an Echino- 

 derm. This opinion of Klein has been reproduced in later times by 



Belemnite restored. 



* ' Phragmoeone.' 



\ Tlie term alveolus has been given improperly to the contents of tlie socket, 

 viz, to the ' phragmoeone.' 



