MADREPORARIA. 785 



XX. Genus Prionastraea. 



The genus is in its definition a most unsatisfactory one. It shows many resemblances 

 to Goniastraea, from which, owing to budding sometimes taking place close to the columella, 

 individual specimens can only be separated with difficulty. It assumes, indeed, the same 

 position in respect to it as Orbicella does to Astraea. 



The species in the collection shows very distinct variation, which is mentioned under each. 

 In all characters it seems to be very largely correlated with environment, i.e. vegetative. The 

 edges, the upper sides and lower parts perhaps of the several species, differ somewhat from 

 one another in the sizes, depth, thickness of walls, spinulation, etc. of the calices. Yet the 

 colonies of P. crassior and P. fusco-viridis clearly show that there are limits to this variation, 

 and that it follows along distinct lines for the several species. The two types of calices 

 described in P. robusta and P. magnistellata are also in this respect remarkable. The 

 number of the septa is fairly variable for each species, but the number which run into the 

 columella is about the same in all (18 — 24) owing to physical causes. The septa preserve 

 their type in each species, though this shows considerable variation in P. fusco-viridis, 

 apparently in this case largely connected with the several localities from which the specimens 

 were obtained. 



Distribution. A common reef-builder, great masses on the outer slope to 15 /, small 

 colonies on the reef-flat even under the breakers, very common on lagoon shoals, often forming 

 overhanging masses ; vertical to 30 fathoms. Colour, generally brown to green. The colonies 

 tend to be much bored into by worms and other destructive organisms at the base, but 

 the surface is usually entire. 



60. Prionastraea pentagona Klz. Klz., p. 39, iv. 7, and X. 5. 



A small .sjDecimen, 8x6 cm., from the breaker zone of the reef at Minikoi, closely 

 resembles Klunzinger's description and figures of this species. The calices are rather smaller, 

 averaging about 6'5 x 5 mm., largest 8'5 x 5 mm. 



61. Prionastraea spinosa Klz. Klz., p. 39. 



A small specimen from S. Nilandu, 30 /'., with a dozen corallites answering in every 

 respect to the above species save that it would appear to be in its structure rather more 

 delicate. The septal sides are a little rough and spiny. 



62. Prionastraea robusta (Dana). (PL LXIV. fig. 42.) Dana, p. 248, xiii. 10. 



A specimen similar to P. crassior in shape, with similar rough-sided septa with blunt, 

 spiny teeth (a little higher and more regular than Dana's fig. 10 b), with nearly solid walls, 

 endothecal dissepiments about "5 mm. apart (in longitudinal section 3 cells in 2 mm.), not 

 very oblique. The columella is one-fourth to one-fifth the diameter of the calice, almost 

 flat-topped, dense, very finely ti'abeculate, spongy, and the innermost teeth of the septa, 

 especially in the deeper calices, simulate a paliform crown. 



The specimen is part of a mass, one side and much of the top of which had been 

 killed apparently by sediment, while the other side was growing out vigorously underneath, 

 with a longitudinally striped, almost costate, lower surface, with no distinct epitheca such 

 as is generally found in the genus. Budding takes place near the centre of the calice so 

 as almost to simulate fission, a character considerably enhanced by the rapid formation of 



