104 THE VOYAGE OF H.M.S. CHALLENGER. 
2. Prionastrea robusta (Dana). 
Astrxa robusta, Dana, Zoophytes, p. 248, pl. xiii. fig. 10. 
The cells on the apical parts of the lobes are deep, with very thin walls and narrow 
septa; on the basal parts the cells become shallow, with broader septa and thicker walls. 
The species is very close to the Prionastrea abdita and probably will have to be 
united with it. 
Locality —Amboina. 
3. Prionastrea obtusata, Milne-Edwards and Haime. 
Prionastrxa obtusata, Milne-Edwards and Haime, Cor., 1. p. 518. 
The extremely narrow and thin septa, which appear as mere denticulate striations on 
the thick and compact walls, give a peculiarly open and naked appearance to the calicles. 
A fifth eycle is generally represented, but remains very imperfect and small. 
Locality.—Kandavu, Fiji. 
4. Prionastrea quoyi, Milne-Edwards and Haime. 
Prionastrxa quoyt, Milne-Edwards and Haime, Cor., ii. p. 519. 
The walls in this species are very unequal. In many of the outer cups the walls are often 
from 3 to 4 mm. wide, and distinctly sulcate, and the septa are firm and rather thick; but 
generally the walls are thin, from 1 to 2 mm. wide, scarcely or not at all suleate, and the 
septa are very thin, with subtrabeculate teeth. The fourth cycle is imperfectly developed. 
Two specimens were collected. 
Locality.—Reefs, Fiji. 
Genus 22. Plesiastrwa, Milne-Edwards and Haime. 
Plesiastrxa, Milne-Kdwards and Haime, Cor., ii. p. 489. 
5 Duncan, Rev. Madrep., p. 107. 
This genus is extremely close to Orbicella, from which it is distinguished by the presence 
of pali. In those species of Orbicella, in which a strong paliform lobe is developed, it is 
a matter of great difficulty to distinguish these structures from the pali of Plesiastrea. 
Two species are in the collection. 
1. Plesiastrea urvillei, Milne-Edwards and Haime. 
Plesiastrea urvillei, Milne-Edwards and Haime, Cor., ii. p. 490. 
Of this species a small flattened specimen occurs, which differs from the ordinary 
form in having the calicles placed more widely apart, and the septa more prominent. 
The pali are generally unequal. 
Locality.—Kandavu, Fiji. 
