32 ■ THE GENUS MONTICULIPORA. 



droid species, the corallum may be readily differentiated into 

 two distinct regions, an axial ?i.\\6. a peripheral region. In the 

 axial region the corallites are generally thin-walled and angular, 

 often with few tabulae. On the other hand, in the peripheral 

 region, the corallites are often thick-walled, commonly rounded 

 in shape, and usually with many tabulae ; while it is in this 

 region that we meet with " interstitial tubes " and " spiniform 

 corallites," if these structures are present at all. There are, 

 however, a few types in which the corallum does not exhibit 

 this differentiation into a deep and a superficial region. The 

 relations of these two regions to one another when developed 

 at all, necessarily varies with the form of the corallum. The 

 commonest arrangement is that found in the dendroid species 

 of Montienlipora, where the axial region of the corallum is 

 truly axial, and forms a central fasciculus of nearly vertical 

 thin-walled tubes ; while the peripheral region forms a thicker 

 or thinner investment to the median axis, and is composed of 

 the tubes of the latter as they become bent and curved out- 

 wards at varying angles in order to reach the surface. 



It is most important to remember this common difference in 

 the characters of the axial and cortical portions of the corallum 

 in the Monticuliporoids, because it is impossible otherwise to 

 prepare thin sections of such forms which will give any adequate 

 or correct idea of their structure. If, for example, we take any 

 dendroid type of the genus, where the corallum is composed of 

 an axis of nearly vertical tubes, and a cortical region composed 

 of the same tubes bent at all angles up to 90" from the vertical, 

 it will be at once obvious what sections it is necessary to make, 

 and also what any given section will show us. Thus a longi- 

 tudinal or vertical section, taken through the median plane of 

 the corallum, will give us a vertical section of the tubes of the 

 axis, and will also divide longitudinally the outwardly-bent 

 and curved tubes of the cortical portion. On the other hand, 

 a genuine transverse section, taken at right angles to the long 

 axis of the corallum, will give us a true cross-section of the 

 corallites in their axial region, where they are nearly vertical ; 



