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THE VOYAGE OE H.M.S. CHALLENGER. 
of Retepora imperati from the Mediterranean. The points of difference, as will be seen, 
between them are very slight. The operculum of Retepora imperati is smaller and the 
muscular insertions on the sides of the ring are above the middle, whilst in Retepora 
elongata they are below it. But these differences, trifling as they may seem, appear to 
be quite constant. 
In the general form of the zoarium the aspect of the two differs considerably. The 
arctic form being altogether much more robust and the fenestras of larger size and very 
unequal ; in fact, in some instances specimens of Retepora elongata might be referred to 
the flamboyant or reticulate group, whilst in Retepora imperati the fenestrse are remark- 
ably uniform in size and shape, and the whole growth is typically fenestrate. Notwith- 
standing the differences above noticed, and considering the wide interval of space in the 
habitats of the two forms, which would render variation extremely probable, I am much 
disposed to consider Retepora imperati as a variety of Retepora elongata. 
(5) Retepora tessellata (PI. XXVII. fig. 8). 
Retepora tessellata , Hincks, Ann. and Mag. Nat. Hist., ser. 5, vol. i. p. 358, 1878, pi. xix. 
figs. 9-12; Macgilliv., Proc. Roy. Soc. Viet. 1883. 
Character . — Zoarium large, intricately folded and contorted in a very irregular 
manner. Fenestrse oval or circular, small, tolerably uniform, but occasionally larger 
in parts of the zoarium. Zocecia (young) urceolate, convex. Orifice semicircular, widely 
sinuated or notched (no fissure) in front. Peristome thick, with four large articulated 
spines (fig. 8 d), two behind and one on each side. In the older zooecia the orifice is 
deeply immersed and the two lateral spines only remain (fig. 86). Surface generally 
very rugose and studded all over with minute avicularia with an acicular mandible ; 
besides which, sparsely distributed, are large rostriform avicularia, with an acute lanceolate 
mandible with a very broad base (fig. 8 f) and pointing downwards. Dorsal surface very 
rugose, often coarsely pitted and studded with innumerable small acicular avicularia, like 
those on the front. Ooecia deeply immersed, cucullate. 
Habitat. — Simon’s Bay, Cape of Good Hope. 
The extraordinary way in which this species bristles with minute avicularia on both 
aspects is a very distinctive character. As only a few small fragments occur in the 
collection, no certain indication of the general habit of the zoarium is afforded. But 
a very good specimen exists in the Oxford Natural History Museum. 
