Fistulipora iucrustans, PhilL (F. minor, M'Coij). 241 



has grown at the surface over the cells. That this secondary 

 deposit of sclerenchyma covers the successive growths ^ of 

 mesopores in F. incrustans in thicker and thinner layers is 

 well seen in transparent sections of its zoarium, where it 

 presents an entirely different structure from that forming the 

 true cell- walls. In the latter the calcite is seen to be much 

 more dense, and transmits less light through its structure 

 than that of the sclerenchyma-layers. This difference of 

 appearance in the two layers is very observable in nearly all 

 my sections and enables them to be readily distinguished by 

 the greater transparency of the one compared with the other. 

 I first noticed this closed condition of the mesopores on the 

 outer surface of specimens, and thought the calcareous layer 

 was confined to such surface ; but sections of the organism 

 show that it is present throughout the whole thickness of 

 the zoarium, from its early incrusting condition to its later 

 stages, where it is seen to increase in the thickness of its 

 layers in many specimens as they reach the outer surface. 

 Vertical and tangential sections show that it is confined chiefly 

 to the interspaces between the autopores, but it is also seen to 

 coat the outer surface of the tube- walls of the autopores, and 

 in their earlier reclined condition, before becoming erect, the 

 sclerenchyma is seen to rest as a thick layer upon the outer 

 concave curve of the tubes, where it forms at the surface of 

 the zoarium the thickened raised lips of the cells that ai-e so 

 noticeable in the younger stages of the organism, [n the 

 older stage, in which the autopores become more erect and 

 with circular mouths, the sclerenchyma forms a regular layer 

 around the walls, as well as often a thin deposit in their 

 interior. These layers of sclerenchyma, whether resting upon 

 the mesopores or autopores, are seen to be of very variable 

 thickness throughout the structure and extent of the same 

 specimen, and differ much even in examples from the same 

 locality. In vertical sections some of the layers are not 

 thicker than the true cell-walls, which are always thin. 

 Other layers, however, are seen to be from ten to fifty times 

 this thickness ; these are seen in transverse sections to 

 form zonal bands around the circumference of the zoarium 

 and mark successive stages in the growth of the mesopores 

 outwards towards the surface. 



Another interesting character shown in vertical sections 

 and illustrating the contemporaneous growth of the scleren- 

 chyma-layers witli that of those forming the cell- walls, is 

 where we see that every successive group of mesopore-cells 

 is completely separated Irom those below by the sclerenchyma- 

 layer, the amount of separation in the true walls in each 



