MORPHOLOGY OF THE CORAL SKELETON. 39 



gence. When the septum is viewed with the edge toward one, the strife are 

 seen to be perfectly symmetrical with reference to the median plane of the 

 septum, one stria standing exactly opposite another. Along the course of 

 each stria are some rather pointed granules. Each pair of opposite striae 

 ends on the septal margin in a distinct, rather pointed, tooth. 



The septum maybe divided into two parts: First, that witliin the wall, 

 the intrathecal portion, or what is usually understood by the septum ; and 

 second, the extrathecal portion, or what is usually called the costa. The 

 inner ends of the septa become modified into lobes, called pali. 



SECTION PARALLEL TO FLAT SURFACE OF SEPTUM. 



(PL I, fig. 2.) 



Such a section passes radially through the longitudinal axes of the 

 striae and permits a study of their minute structure. The distance across 

 the stria> is not alwavs the same. They are narrower at the line of diver- 

 gence, and become wider as they pass away from it. As the septal edge 

 is approached, new strife are quite often introduced between the old. The 

 distance across four striae near the line of divergence measures 0.655 mm., 

 or an average of about 0.16 mm. At the edge one stria may measure 

 0.374 mm. across. Along the axis of each stria is a series of darkish or 

 clouded bodies, and crossing the axis are angular lines with tlie apex of the 

 angle directed distally. A pair of opposed striae is a septal trabecula; the 

 dark bodies along the axis are along its axis of calcification; the lines alluded 

 to as crossing the trabecular axis are the lines of trabecular growth, aud are 

 formed parallel to the growing edge of the trabecula. Between the trabec- 

 uljB are dark lines indicating the lines of their fusion. Examined more 

 closely, minute calcareous fibers that diverge distally from the axis of the 

 trabecula are seen. The dark line of the trabecular axis is composed of a 

 series of roundish bodies — there are about three in the space of 0.032 mm., 

 although they may be slightly more crowded. The fibers converge prox- 

 imally to these dark points, which mark the axis of the calcification. 



CROSS SF.CTION OF SEPTUM. 

 (PI. I, fig. 3.) 



In this section the trabeculse are cut across, and one looks down upon 

 the ends of their axes. There is no necessity to give measurements of 

 the distance from one axis to the next, as, unless the trabeculai are cut 



