582 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Genus MffiANDRINA Lamarck." 



Polyps verrucose, incompletely separated, forming' mainly continuous, meandriform discal and 

 columnar systems, and giving rise to massive, convex, gibbose, subplane or subspheroidal colonies, fixed 

 by a comparatively narrow or broad base; perithecal continuation of the gastro-ccelornic cavity and 

 mesenteries at the margin of the colony. Contiguous column walls united along a narrow, common, 

 thecal ridge; on retraction capable of folding over the tentacles and covering the lateral margins of the 

 disk; no sphincter. Tentacles in two, alternating, slightly entacmaeous rows, entocoelic and exocoelic, 

 introvertible, short, rounded terminally, stem with irregular urticating areas. Disk with numerous, 

 closely arranged, small oral apertures. Stomodaeal walls deeply ridged. 



Mesenteries all complete, with occasional incomplete developing pairs, arranged in irregular stomo- 

 daeal systems; all filamentiferous; directives absent; increase irregiilarly by addition of single unilateral 

 pairs. Mesenteries and filaments protrusible. Septal invaginations entocoelic and exocoelic. dicyclic, 

 interseptal loculi incompletely separated below. 



Asexual reproduction by continuous incomplete fission. Polyps monoecious. 



Example. — Mseandrina labyrinthica (Ell. & Sol.). 



1VL/EAXI>RIXA LABYRINTHICA (Ellis A Solander). 



(Pis. XX-XXI1, figs. 138-147.1 



External characters. — The colonies are massive, subspheroidal, the upper .surface uniformly 

 rounded, not thrown into gibbosities. The species occurs in abundance on the reefs around 

 Jamaica, often forming blocks several feet across, and fixed by a narrow, irregular base; even 

 large colonies are free or are readily detached when collecting. 



The external appearance of the living colonies varies greatly according to the condition of 

 expansion or retraction of the superficial soft tissues. The general relations during partial 

 retraction arc best conceived of as a double system of meandriform depressions, separated by a 

 fringe of short tentacles. One series of grooves is formed by the continuous, narrow, discal areas. 

 the other by the united column walls of two contiguous polypal systems. The discal troughs are 

 much deeper than the columnar, and occur within the united calices; the slight columnar depres- 

 sions run along the common thecal edges or collines. and the line of union of the two adjacent 

 column walls is clearly indicated at the bottom. 



On full expansion the superficial tissues become distended to such a degree that they are 

 raised for several millimeters above the skeleton, the discal region increasing from '1 to S mm. 

 across. Along the line of union the two contiguous column walls remain affixed to the skeleton, 

 so that the walls on each side rise almost vertically from the line of attachment, and may actually 

 apply themselves to one another by their outer surface, or remain separated only by very steep 

 valleys. In the former ease the two contiguous fringes of tentacles intermingle in such a way 

 that the whole surface of the colony presents to view little more than the. enormously enlarged, 

 convex discal areas. During maximum expansion the tentacles and the verruca 1 may almost 

 entirely disappear, the walls of the former becoming part of the Hat discal tissues. In the 

 fully retracted state the appearances are reversed; the deeper valleys are now formed by the 

 discal areas, and the ridges by the columnar expansions resting on the septa. The column may 

 become partly overfolded, and under some conditions the two overfolding walls connected with 

 each discal system may extend horizontally, and almost come into actual contact, so as to 

 completely hide the tentacles and disk. 



The column wall appears thick and opaque when the polyps are retracted, but more delicate 

 and transparent when fully expanded. The surface exhibits verrucose ridges and smooth 

 furrows, cori'esponding respectively with the internal septal and mesenterial divisions. The 



« "Colony massive, dense, convex, gibbose, subplane or subspheroidal, largely fixed by its base. The series of 

 corallites unite by their walls, which are compact, and produce long, simple ridged collines. The valleys are 

 sinuous, long, but vary in length, depth, breadth, and meandroid nature. Calices mostly indistinct, some may he 

 circumscribed. Columella formed by masses of spongy tissue well developed. The septa are close, parallel, their 

 inner edge thickened ami enlarged transversely; upper margin denticulate, moderately granular laterally. Union of 

 the transverse enlargements of neighbouring septa near the columella often occurs, and gives a paliform appearance. 

 Endotheca and epitheca exist." (Duncan, L885, p. 88. ) 



