40 EOCEJJE AND LOWER OLIGOCENE CORAL FAUNAS. 



exactly perpendicular to their axes, the measuremeuts will Iw niisleading. 

 The figure well shows the appearance of the section, but much of it has 

 no structural significance. The axis of the trabecula in cross section is 

 either a dark- or a light-colored point. Surrounding this point is a con- 

 centric light-colored area, which is composed of calcareous fibers that radiate 

 outward, like the spokes of a wheel, around the central point. In many 

 instances the fibers converge at one point, but in some cases there seem to 

 be several points, two or three, very closely crowded together, each with 

 its group or fascicle of fibers. Thu first condition is the usual one; the 

 latter is the exception. Running concentrically around the centers and 

 crossing the fibers are very fine, alternately darker and lighter bands — the 

 growth lamella?.' Outside of this lighter area and surrounding the trabeciilar 

 axis the figure sliows a darker area. This darker area has no structural 

 importance. The difference in color is due simply to difference in condition 

 of preservation. A suture indicating the line t>f fusion lietween adjacent 

 trabeculse can be distinctly seen between nearly all the trabeculte. The 

 granulations on the surface are produced by some fibers passing outward 

 beyond the general surface of the septum. 



All septa whose structure has been studied have been found to be com- 

 posed of trabeculae more or less completely fused. The various methods 

 of trabecular formation will be briefly outlined. 



As yet it has not been determined to what features of structure the 

 greatest systematic weight should l)e given. 



THE AREA OR LINE OF DIVERGENCE. 



It was pointed out in describing the structure of the Manicina areolata sep- 

 tum that the trabecular on one side of the line running parallel to the longi- 

 tudinal axis of the septum bend toward the interior of tlie corallum, while 

 on the outside of this line they bend toward the outside. The width of the 

 zone on either side of this line is variable, and in some instances the exter- 

 nal one may be very narrow or suppressed altogether. The external zone is 

 usually narrower or suppressed in coral> that have a small costal development, 

 with nonexsert septa, or those with septa rouHuent from one calice to the 

 next. The outer zone is very narrow in Flabellum, and in Mesomorpha 

 duncani (PI. XVIII, fig. 10) there is no line of divergence for an individual 



I Cf. Ogilvie, op. cit., pp. Ill, 112. 



