116 THE VOYAGE OF H.M.S. CHALLENGER. 
5. Pavonia pretorta, Dana. 
Pavonia prxtorta, Dana, Zoophytes, p. 325, pl. xxii. fig. 5. 
Some fine specimens and small fragments of this species were obtained. It is much 
more delicate than Pavonia cactus. The septa of the apical calicles are very unequal 
both in this species and in Pavonia formosa, which greatly resembles it; in the more 
basal calicles the septa are subequal. 
Locality.—Tahiti. 
Genus 4. Agaricia, Lamarck. 
Agaricia (pars), Lamarck, Syst. Anim. sans Vert., p. 375, 1801. 
Undaria et Mycedium (pars), Oken, Lehrb. der Naturg., Zool., i., 1815. 
Agaricia et Mycedium (pars), Milne-Edwards and Haime, Cor., il. pp. 72, 80. 
Agaricia et Mycedium, Duncan, Rev. Madrep., pp. 158, 161. 
The characters on which the genus Mycediwm has been maintained to be separate 
from Agaricia seem to me altogether insufficient for generic distinction, and, bearing in 
mind the great variation which takes place in the concentrically seriate arrangement and 
circumscription of the calicles in the species which have been placed under these genera, 
I believe it will be impossible to find any fairly constant character by which to 
separate them. A careful comparison of a series of specimens of Agaricia agaricites, 
Agaricia frondosa, Agaricia undata, Agaricia lamarchi, Agaricia (Mycedium) fragilis, 
Agaricia (Mycedium) elephantotus,' will show all stages between the concentrically seriate 
arrangement of the scarcely circumscribed calicles and the more or less scattered arrange- 
ment with well circumscribed cups. This variability in the arrangement of the calicles 
is well shown even by individual specimens of the same species, Agaricia (Mycedium) 
JSragilis furnishing a typical case, as pointed out by Pourtales.* So completely transitional 
is this species between the more extreme forms of the genus, that Dana himself, while 
he places it in the subgenus Mycedia, states,’ in his description of the species, that it 
“has much of the habit of Agaricia wndata, and might with equal propriety be placed 
among the Undamz#” (Agaricia proper). 
The Mycedium of Milne-Edwards and Haime, besides Agaricia elephantotus, con- 
tained three other species, two of which are forms of the genus Phyllastrea, Phyllastrea 
tubifex, and Phyllastrea okeni; and the other apparently an old form of a species of 
Leptoseris, Leptoseris elegans. The genus Phyllastrea is well distinguished, both by 
its spongy columella and by its distinctly scattered calicles, the well-developed, raised 
wall of which is very prominent on one side and is not due to flexure of the lamina nor 
to the fusion of synapticular structures. The genus Leptoseris was founded on a young 
1 This specific term, which was used by Pallas (Elench. Zooph., p. 168), and which has been retained in the same 
form by later writers, would seem to have been a misprint for “elephantopus.” 
2 Illus. Cat. Mus. Comp. Zool., Cambridge, U.S.A., No. iv. 
’ Zoophytes, U.S. Expl. Exped., p. 341. 
