218 Christmas Island. 



secondary septa are long and thin, the latter being considerably the 

 shorter ; both are connected to the columella, though in sections 

 the connection maj- not be shown in all cases. 

 Distribution. — Christmas Island. 

 Fossil : 



No. 946. From the limestone pinnacles a little south 



of the middle of the island ; alt. 550 feet. 



No. 511. A specimen fi'om top of the upper cliff, 



middle of the east coast. 



Figure. — PI. XIX, Fig. 4. Part of a transverse section of 



specimen No. 946, x 2 dia. 



Affinities. — Orbicella prceheliopora is a species intermediate in 

 characters between the Orbicella irregularis (Mart.) from the 

 Miocene of Java and 0. heliopora (Lam.) from the Australian 

 seas. It is probably nearer the foiTaer, which differs by having 

 only four cycles of septa, and by having the corallites crowded 

 together so that they are in-egular and polygonal in shape. The 

 two species agree in the irregularitj' of the septal sequence. In 

 two adjacent sectors in one corallite of the type-specimen of 

 0. pr(sheliopora the sequence is as follows : — 



1 4 3 2 5 3 1 

 16473525341 



According to Martin's diagram of 0. irregularis, the sequence 

 in three sectors in that species is : — 



13 2 3 1 



14 3 5 2 3 1 

 1 3 5 2 5 3 4 1 



From 0. heliopora the Christmas Island coral differs by having 

 a looser columella and less regular septal sequence ; for 0. heliopora 

 is said to have four complete cycles. The primary, secondary, and 

 tertiary septa are said, moreover, to be not very different in size 

 in the living species. 



Acanthastrsea patula (Dana). 



Orbictlla patula, Dana, 1848. Zoopli. : Expl. Eiped. "Wilkes, vol. viii, p. 209,. 



pi. X, fig. 14. 

 Acanthattraa ? patula, Edwards & Haime, 1857. Hist. nat. Cor., vol. ii, p. 505, 



Yar. paucidentata, nov. 



The genus Aca7ithastr(ea is represented in the collection by 

 a small fragment 70 mm. long ; the corallites are fi'om 9 to 12 mm. 

 in diameter, the majority being 12 mm. The columella is small, 

 and consists of a few denticles, though sections at first sight 

 appear to have a long columella owing to the abundance of central 



