102 THE VOYAGE OF H.M.S. CHALLENGER. 
distinguishable by the nature of the wall, the septa, and the pali. The characters of the 
pali will readily separate it from all other species. 
A very fine but rather small specimen was collected. 
Locality. —Banda. 
8. Gonastrea laxa, n. sp. (PI. III. figs. 4-4). 
Corallum incrusting, forming broad, convex spreading masses. Calicles oblong or 
polygonal, from 6 to 8 mm. in the long diameter, and from 3 to 5 mm. deep; walls 
generally quite thin and acute, rarely flattened and wide; septa very unequal, wide 
apart, of three cycles, those of the third cycle very small and often quite rudimentary, very 
seldom there are rudiments of a fourth cycle in some of the largest calicles ; from about 
seven to nine large septa reach to the centre, surmounted by small, long, upright pali, 
which are scarcely distinguishable from paliform teeth, and surround a central depression 
in which there is no trace of a columella. The septa are exsert, slightly thickened at 
the wall, and finely and raggedly denticulate, giving a rough appearance to the corallum. 
Endotheca very abundant, visible above between the larger septa; the dissepiments small, 
very thin and close. 
This species presents an interesting modification of the structure of the wall, the 
thick and flat form showing a very decided approach to the genus Astrea. 
The species has also a very great resemblance to forms of the genus Prionastrea, 
and this is greatly increased by the slight development of the pali, which sometimes do 
not seem to differ in any respect from large paliform teeth. The increase by fission 
seems, however, to be evident in many parts of the corallum, though it is, confessedly, 
difficult in the dried condition to distinguish this from sub-central, intra-calicinal gemma- 
tion. It is possible, on this account, that this form will have to be referred to the genus 
Prionastrea when living specimens of the species have been examined. 
Two specimens, one a fine and large one, were collected. 
Locality.—Api, New Hebrides. One of the four most abundant reef-forming species 
in this locality. 
Genus 20. Acanthastrewa, Milne-Edwards and Haime. 
Acanthastrxa, Milne-Edwards and Haime, Cor., ii. p. 501. 
* Duncan, Rev. Madrep., p. 119. 
Acanthastrea irreqularis, n. sp. (Pl. IV. figs. 2-2a). 
Corallum spreading, forming a thickened, convex plate. Epitheca well developed to 
the very edge, slightly wavy. Calicles unequal, subcireular, oblong or polygonal, 
