220 Miss M. M. Ogilvie — Microscopic and Systematic 



explanation of the so-called " dark streaks " and other 

 appearances in tlie skeleton. 



The finer structure of the septa will be readily unclerstopd 

 when it is remembered that the septa are formed within radial 

 invaginations of the aboral body-wall of the polyp. The 

 septal surfaces are practically casts of the two flaps of a septal 

 invagination. When the flaps are smooth the calcareous 

 lamellae are also smooth ; but, according to the author's obser- 

 vations, that is seldom the case in recent Madreporaria. 

 Usually the septal flaps are pitted and goff'ered, resembling 

 the pleated muscular flaps of the mesenteries, and the septal 

 surfaces are correspondingly granulated and fluted. The 

 author has found that in all cases the calcified calicoblasts of 

 successive lamellae are grouped around definite centres of 

 deposit situated in the median ])lane — corresponding to the 

 growing edge — of the septum. Subsequently the individual 

 groups of calicoblasts assume the form of radiating bunches 

 of fibro-crystalline aragonite, passing outwards from the 

 original centre of deposit in the median plane to the surface 

 of the septum. The author has given tlie name of "fascicles" 

 to these fibro-crystalline " bunches," and has demonstrated 

 the relation which they bear to the external sculpturing 

 observed on lateral septal surfaces. The emergence of a 

 fascicle at the surface gives rise to a granulation. The fascicles 

 are, however, of varied size; if large, one fascicle usually 

 corresponds to one granulation, if small, a number of fascicles 

 may, in the course of septal thickening, coalesce to form a 

 broad nodular granulation. The size of individual fascicles 

 depends on the original closeness of the " centres of calcifica- 

 tion " at the septal edges. 



The trabecula ( = "poutrelle," Edw. & H.) of a "perforate" 

 septum is composed of symmetrical groups of fascicles '^Xo.ce.di 

 in vertical series. The author has further found that those 

 septa described by Edwards and Haime as " imperforate and 

 leaf-like " are also composed of trabeculse. But the individual 

 parts of these trabecular have an opposite jmir oj fascicles, 

 instead of an indefinite number. The majority of Astrseid 

 genera have septa in which both kinds of trabeculse occur. 

 The author applies the term " simple trabecula " to a 

 trabecula made up of successive pairs of fascicles, as the axis 

 of deposit is in part or wholly common to the opposite 

 fascicles, and the term " compound trabecula " to one made 

 up of successive groups of fascicles. In tlie former case the 

 fibro-crystalline deposit may be said to be bilaterally sym- 

 metrical in the opposite halves of a septum ; in tlie latter the 

 fibro-ciystalline deposit is radially symmetrical around ideal 



