MORPHOLOGY OF THE CORAL SKELETON. 43 



and transverse sections. When the trabecular axes are very close together 

 and the septa are quite thick, after passing a slight distance away from the 

 axis, by simple mechanical crowding, the fibers must necessarily be almost 

 parallel one to another, and near the area of divergence they stand perpen- 

 dicular to the median septal plane; exterior to the area of divergence they 

 diverge outward, interior to it they diverge inward. It seems to me that 

 the arrangement of the fibers of a septum is a function of the size of the 

 trabeculjp, thickness of the septa, and the trabecular inclination, and that 

 it has no importance except when taken as an index to some other character 

 of the septum. 



COMPACTNESS OF SEPTA. 



If the trabeculse composing a septum are continuous and fuse com- 

 pletely, a cojnpact septum is the result ; but sliould the continuity of a 

 trabecula be broken, or should the fusion between adjacent trabeculae 

 be incomplete, septal perforations are produced. The greater number ot 

 corals described in this paper have solid septa. PI. XXI, fig. 7, Eitpsammia 

 elahomta, and PI. XXIII, fig. 3, Turhinaria (?) alahamcnsis, show corals 

 with perforate septa. Pratz^ has described in detail the structure of several 

 perforate septa, viz, Cyclolites, Astrjeomorpha, Leptophyllia, and his so- 

 called Thamnastrsea. We may inquire a little more closely into the causes 

 which produce a compact or perforate septum. In all of the septa whose 

 structure I know the fibro-crystals as seen in longitudinal, tangential, and 

 radial section pass outward from the median septal plane and converge 

 proximally (downward) along the trabecular axis. In order that the fibers 

 of adjacent trabeculse may unite, they nuxst possess a certain lateral elon- 

 gation as they diverge from the trabecular axis. Very often the fibers 

 bending outward from the trabecular axis must be longer than those passing 

 along the axis to the next calcification center. If the calcification centers 

 are elliptical or elongate in the direction of the axis of the trabecula, complete 

 fusion will be fiicilitated. Among some corals^ the fibers are often not 

 long enough laterally to meet those of the next trabecula. The consequence 

 is compact trabecular, whicli fuse where calcification centers are opposed, 

 but pores are left in the interspaces between the trabeculae where 

 no calcification centers stand opposite. When septal pores are formed in 



I Op. et loc. cit. 



'Cf. Pratz'8 fig. 4 of a septum of Cyclolites, section parallel to the flat surface of the septum. 



