496 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



Genotype. — Flabellopora elegans D'Orbigny, 1852. Recent. 



Structure. — The zooecia are short, hexagonal prisms surmounted by 

 two hexagonal pyramids one of which (exterior) is more or less poten- 

 tial. The transverse section shows therefore regular hexagons; the 

 longitudinal section shows elongated hexagons and the meridian 

 section exhibits hexagons enlarging toward the zoarial periphery. 

 The zooecial walls are very thick and in common with each other; the 

 zooecia are therefore not detachable since they have no individual 

 walls. Moreover there is no basal lamella for the same reason and the 

 two juxtaposed zoarial lamellae are perfectly and inseparably united. 



The zooecia are superposed and not juxtaposed as in the other Cheil- 

 ostomata. This arrangement obliterates the apertures, so the latter 

 instead of being arranged on the frontal are located on the distal wall. 

 Moreover the anatomical arrangements being constant in all the 

 cheilostomes, the aperture is by force eccentric and the sinus, the 

 orifice of the compensatrix, is distal. Figure (B) shows the section 

 through an ordinary cheilostome zooecium while figure (C) is a section 

 through a zooecium of Flabellopora. It is easy to see that the special 

 arrangement of the aperture is the consequence of the superposition 

 of the zooecia and of a different mode of gemmation. In these two 

 drawings, in order to be better understood, we have projected on the 

 plane of the figure the aperture which is normally placed on the 

 perpendicular plane. 



The visible part of each zooecium is its distal wall, that portion 

 which is usually hidden in the other Cheilostomata. Although this 

 wall bears the aperture, it bears also avicularia, the adventitious 

 organs of oxygenation absolutely indispensable to zooecial life. 15 

 They are very numerous and arranged geometrically between oblique 

 rows of apertures (fig. D). By rubbing away the surface in order to 

 examine the interior the very special arrangement of the apertural 

 orifice can be easily verified; it is always eccentric and the sinus is 

 immediately adjacent to the frontal wall. In this section that which 

 is visible at the bottom is the distal wall and the vertical walls are the 

 frontal and dorsal walls. This is an arrangement absolutely opposite 

 to that observed in other Cheilostomata (fig. B). However, if the 

 anatomical arrangements are quite identical, there is necessarily a 

 slight displacement in the disposition and the basal insertions of the 

 opercular muscles. 



A particular character in Flabellopora and Conescharellina is the 

 presence of a proximal pore. It is very small, little deep, oblique, 

 opening in the pcristomie, excavated in a single parietal thickness. 

 Very often it is united to the aperture from which it is scarcely 

 separated. The latter then assumes a deceptive schizoporid form, 



» They are lodged precisely in the free intervals arising from the narrowing of the theoretical pyramid 

 which surmounts externally the hexagonal zooecial pyramid. 



