130 EXPLANATION OF PLATES. 



PLATE it. 



FIG. 66. Transverse section through a single trabecula, penetrated by filamentous boring alga, and 

 showing the growth lamellae and their fibrous character. 



FIG. 67. Vertical section through a part of two adjacent septa. The septa are made up of trabeculse 

 arranged in a radiating manner; they diverge from a continuous median trabecula which represents the 

 boundary between the two adjacent calices. The different appearances presented by the various trabec- 

 ulae are dependent upon the part included in the section. Where the section passes along the middle of 

 a trabecula the interrupted dark centers of calcification are seen ; elsewhere only the diverging bundles of 

 fibre-crystals appear. The growth lamelltB are not shown (cf. fig. 68). New trabecuU-e are intercalated 

 at intervals. The dark circular and oval patches are sections of synapticula. 



FIG. 68. Terminal portion of two trabecuhe as seen in longitudinal radial section, that is, parallel to 

 the surface of the septum, more highly magnified than in the previous figure. The section passes along 

 the middle of the trabeculae so that all the dark centers of calcification are seen. In the corallite from 

 which the section was taken the growth lamellae were very distinct, the boundary of each being indicated 

 by a dark granular deposit similar. to that at the centers of calcification. The growth lamellae are arranged 

 parallel with the toothed edge, that is, with the calicoblast layer which secretes them, while the fibro- 

 crystals making up the lamellae are arranged at right angles. The figure should be compared with fig. 

 66, which shows a trabecula in transverse section. 



FIG. 69. Portion of basal plate and epithecal boundary of a very early corallum. The epitheca is seen 

 in section, and the basal plate is sufficiently thin to allow of the passage of light. The basal plate is made 

 up of more or less distinct granules or scales showing a fibrous structure; the aggregation of darker 

 granules represents the first formation of a septum. 



FIG. 70. Portion of basal plate, septa, and epitheca of a somewhat older eorallum than that of fig. 69. 

 The septa and epitheca are ground down to nearly the level of the basal plate. Centers of calcification 

 are not present in the basal plate and epitheca but are very prominent in the septa. 



