244 
described as different species are distinguished from one ano- 
ther only by characteristics, which are due to strong individual 
variation. I have in my material a number of specimens, which 
closely fit in with the typical form of each of the three described 
species. More or less typical representatives of the three described 
„species“ are represented in fig. 112 (the form robusta), fig. 122 
(the form irregularis) and fig. 114 (the form sluiteri). There are 
also a great many which have some particulars by which they have 
to be identified with one of the three so-called species, and besides 
these other features on account of which they would have to be 
referred to another. In the preceding table the essential characteri¬ 
stics by which the three described forms are distinguished are 
given. 
The perforations of the theca are found in all younger speci¬ 
mens, and also in the older colonies in which the corallum is not 
very thick. When the thickness of the corallum increases the per¬ 
forations are wanting in the greater part of the corallum and only 
some small slits are found in the marginal part. Some specimens 
which have a nearly altogether solid theca possess a distinet cen¬ 
tral calicle of larger size than the secondary calicles and septa, 
which are for the greater part neatly radiating from the centre to 
the periphery. 
In many specimens the costae are quite distinetly visible in 
the greater part of the lower surface whilst in other specimens 
this surface is densely covered with spines, which show no radial 
arrangement. These differences are found in specimens possessing 
all other characteristics ascribed to the form robusta, and the costae 
therefore present no constant peculiarities by which the form robusta 
can be -separated from the form irregularis, a faet which also 
has been stated by van der Horst (1921). According to van 
der Horst the arrangement of the septa is the criterium by which 
the two forms may be distinguished. In D. robusta the septa are 
radiating from the obvious central calicle to the margin of the 
colony whilst in D. irregularis the septa have an irregular course. 
Now in all very young specimens the septa of the originally simple 
corallum are distinetly radiating from the centre to the margin and 
this may remain so after further growth of the colony. In many 
specimens. however, the formation of the great number of secon- 
