SOLITARY CORALS. 195 



Family : FLABELLID7E. 



Simple corals, multiplying asexually by transverse fission from a fixed nurse-stock. 

 Corallum more or less compressed and flabelliform or cuneiform. Calice elongated ; 

 elliptical or angular at the extremities of the long axis. Septa numerous, increasing 

 in number during the growth of the corallum, chiefly by the addition of new septa in 

 the systems contiguous to the dnective septa, in such a manner as to appear to be 

 arranged in a variable number of ternary systems. The columella may be essential 

 and lamellar, or parietal and formed by the union of spines or trabecular projecting 

 from the lower ends of the principal septa. The wall is protothecal, and increases in 

 thickness only by addition to the inside surface. There is no edge-zone. Costa? 

 rudimentary or absent. Protothecal spines commonly present. Genera : Flabellum, 

 Lesson, and Placotrochus, M. Edwards and Haime. 



The Flabellida?, as characterised above, differ from all the other Turbinolidse, 

 among which they have hitherto been placed, in the absence of an edge-zone, that is, of 

 soft tissues external to the wall. The wall, as von Koch pointed out (30), is an 

 " epitheca," or, if we adopt the more exact nomenclature of Bernard, a " proto- 

 theca," that is to say, it is a direct upward continuation of the primitive basal cup 

 common to all Madreporarian corals, of which the development has been carefully 

 described by von Koch (28), de Lacaze Duthiers (33), and Duerden (10). As 

 there are no soft tissues external to the prototheca, it can only increase in thickness 

 by addition from within, and a section of the wall of Flabellum shows that the "dark 

 line of growth," which lies in or near the middle of the theca of other corals, is here 

 on the outside. As the wall does not increase in thickness externally, the costas are 

 very feebly developed, and there is an absence of the ridges, spines, and other 

 external ornamentations formed by the deposit of additional calcareous matter on the 

 outside of the wall by the activity of the calicoblasts of the edge-zone. The spinous 

 processes and rootlets found in most members of the Flabellida? are hollow, and only 

 become solid by the secondary deposit of calcareous matter within. Thus they differ 

 from ordinary costal spines, and their mode of formation, by local extensions of the 

 lip of the calice, has been indicated by de Lacaze Duthiers (33). In all the 

 Turbinolidse, as here limited, there is a distinct and usually well-developed edge- 

 zone. Hence the wall increases in thickness, both externally and internally, and 

 there are well-developed costal ridges, variously ornamented with granules, tubercles, 

 or spines, or sometimes the furrows between the costal ridges may be filled up by a 

 secondary deposit of an " epithecal " character, formed by the calicoblastic layer of 

 the edge-zone. Moreover, in the typical Turbinolidse, the septa grow in height well 

 beyond the limits of the protothecal cup, and are either prominently exsert or their 

 peripheral margins are thickened and become attached to one another, forming the 

 so-called pseudotheca of Ortmann. The marked difference in the structure and 

 growth of the wall seems a sufficient reason for separating Flabellum and its allies 



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