12 BRITISH FOSSIL CORALS. 



commonly met with. In the genus Acervularia the distinguishing of so-called paliform 

 lobes or enlargements and teeth is sufficiently difficult. 



The number of genera without pali is very considerable. 



Columella. — This structure is in the axis of the coral, and may be noticed in the 

 centre of the calice or of transverse sections of corallites, whilst in longitudinal 

 sections it is to be seen passing from the base upwards, having the pali or septa ou 

 either side.' 



The columella is not invariably present, but in some species it forms the most important 

 part of the calicular apparatus.^ The most highly developed columellse spring from the 

 centre of the base of the young corallum, increase in height with the growth of the septa, 

 and always appear as prominent organs in the calice. These columellse grow uide- 

 pendently of the septa, and are not fonned by their internal and free terminations. For this 

 reason they are called "essential" or "jorojona;" they generally assume the styliform, 

 the fasciculate, or the lamellar character, and may or may not have pali attached to 

 them.' 



The second kind of columella is termed " septal," and is produced by the inner ends 

 of the septa dividing into longitudinal " poutrelles." They have a fascicular arrangement. 

 These "septal" columellse are rare, and may, for all practical purposes, be considered 

 with the next kind. 



The third kind of columella is formed by the septa dividing into numerous processes 

 before they approach closely ; the processes unite centrally, and throw out lateral growths, 

 so that a more or less dense, spongy, or cellular structure results. This columella is 

 termed parietal, and may be very highly developed or may be rudimentary. In the 

 latter instance the columella may only be recognised by a slight bifurcation of the inner 

 ends of the septa, with a sparely developed cross tissue. 



False columellcB are formed by the soldering together of the inner ends of two or more 

 septa, by the twisting of the inner ends of several septa, and by the presence of endotheca 

 close to the septal inner margin. 



Rudimentary columella are often observed, which cannot be classified with any of the 

 above ; they may be formed by a lateral junction of the inner ends of the larger septa, by 

 processes connecting them, and by the inner ends becoming clubbed in outline, and more 

 or less irregular in their direction. 



There are many modifications of these varieties of columellae, but their division into 

 essential, septal, parietal, and false, is of great practical value, and they can always be dis- 

 tinguished with care. The calicular terminations of the columellse vary in size, projection, 

 outhne, and arrangement. 



1 Plate I, figs. 5, 6, 10, 18. 2 Plate VII, fig. 12. 



2 Plate I, figa. 5, 6, 8, 10, 12 ; Plate IX, figs. 3, 6, 10. 



