588 .MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



For some distance below the stomodsBum the coelenteron is imperfectly partitioned by the 

 septal invaginations, but toward the base it becomes divided into twelve distinct loculi, each of 

 which contains two unpaired mesenteries. Central to the loculi are sections through the invagi- 

 nations which covered the pali and columella; they are at first free, but below are continuous 

 with the septal invaginations. At first each loculus is partly divided along its peripheral border 

 by the exocoelic septal invaginations, but these disappear in the lower regions, and each loculus 

 is then a simple chamber. The convoluted mesenteries at first crowd the loculi, but afterwards 

 wholly disappear. 



('.—SECTION FUNGACEA. 



MADREPORARIA IN WHICH THE MESENTERIES AND THE BASAL WALL LINING THE INTERSEPTAL 



LOCULI ARE PERFORATED BT SKELETAL BARS. TENTACLES OFTEN SMALL. SIMPLE OR DIMORPHIC, 



WIDELY SEPARATED. 



Family PLESIOFUNGID^E. 



Genus SIDERASTR/EA Blainville." 



Polyps smooth, distinct, form compact, massive, convex or plane, incrusting colonies; united with 

 one another along a common polygonal edge, 'without perithecal continuation of the mesenteries. 

 Column wall smooth, short, not overfolding on retraction. Tentacles small, knobbed, in somewhat 

 irregular cycles, distant from one another; sessile and exposed on retraction; dimorphic — an inner 

 (entoccelic) series bifurcated, an outer (exocoelic) series simple. Stomodseal walls smooth. 



Mesenteries completely or incompletely tricyclic; six pairs complete, two pairs of directives, all 

 filamentiferous, perforated by synapticula. Septal invaginations entoccelic and exocoslic, completely or 

 incompletely tricyclic; incompletely separated for the greater part of their length, and perforated by 

 several longitudinal rows of circular skeletal ingrowths i synapticula). 



Asexual reproduction by intercalary and marginal gemmation. Viviparous. 



Examples. — Siderastrasa siderea (Ell. & Sol.), Siderastrasa radians (Pallas). 



SIDERASTR.EA SIDEREA (Ellis & Solander). 

 i Pis. XXIi-XXIV, figs. 150-160.) 



External characters. — Colonies of this species often form large, massive, compact, subsphe- 

 roidal or incrusting masses on the sea floor about the reefs. The polyps are closely arranged and 

 polygonal in outline at the base: adjacent polyps are united along a narrow, common calicinal 

 wall, so that no periealicular continuation of the column wall and gastro-ccelomic cavity is possi- 

 ble. The polyps in a colony are not disposed in any regular plan; a slight tendency to a linear 

 or circular arrangement is apparent in places, but the intercalation of new individuals at any 

 spot introduces irregularities. 



The polyps do not readily expand, and even when this does take place the superficial tissues 

 are raised only a little above the corallum. and the column is somewhat dome shaped, not assum- 

 ing the regular cylindrical form characteristic of coral polyps generally. Outside the tentacular 

 zone, but not sharply marked off from it, is a very limited, smooth, polygonal area, which is all 

 that represents the column wall during partial or complete retraction. In the latter condition 

 the superficial tissues are deeply depressed, and lie closely over the septa, being thrown into 

 corresponding ridges and furrows (fig. 156). 



The tentacles in partly retracted polyps appear as short, stumpy processes of the disk, widely 

 separated from one another, the tentacular zone occupying nearly the whole of the superficial 

 area of the polyp (tig. 150). Thej^ arc broad at the base, but narrow a little terminally, becom- 

 ing swollen at the apex. The inner entoccelic members are bifurcated distally, and during 

 retraction one moiety is disposed on each side of the underlying septum; the outermost exocoelic 



""Colony massive, convex or plane, dense, incrusting. Corallites united by thin ami often indistinct walls. 

 Calices subpolygonal, deep, margins rounded. Columella small, papillary, made up of ascending trabecule, which 

 often fuse, here and there, into a mass. Septa solid, rather close, thin, denticulate where free, often uniting. Two 

 rows of synapticula close- to the wall unite tin- opposed septal lamellae, and tend to fill up the interseptal loculi near 

 the wall. Septa imperforate. Endothecal dissepiments few. Gemmation submarginal." (Duncan, 1885, p. 134.) 



