50 
PROF. P. M. DUNCAN AND MR. W. P. SLADEN ON TIIE 
basal. This is erroneous, and so is the figure given in bis plate. 
The same specimen shows the very small size, comparatively, of 
the radial plates (oculars). 
It is evident then that the adult forms of Diseoidea cylindrica 
have all the five basals perforated for the madreporite, and all, 
except the fifth, perforated for the genital duct. 
The Ambulacra . — Wright was the first to point out that the 
simple, straight condition of the pairs of pores was not univer- 
sally found in the species, but that the pairs on the actinal area 
might fall into a biserial arrangement. 
Now it appears to us that allowance must be made for the age 
and size of the tests in considering the particular distribution of 
the pairs of pores ; for in the largest specimens the arrangement 
of the pairs differs from that seen in the immature forms, espe- 
cially near the peristome. Again, there is a point which must 
be remembered, and it is that in full-grown specimens the am- 
bulacra are flush with the test above the ambitus and raised 
above its level actinally. The poriferous zones are on the slant 
of the raised surface actinally ; and it is on this slope that the 
greatest crowding of the pairs of pores happens. Above the 
ambitus the simple arrangement of the pairs in one row is inva- 
riable ; and it is seen in small forms even near to the peristome. 
As a rule, the pairs are not crowded above the ambitus; and 
they may be larger there than actinally, where the crowding may 
be considerable. 
The height of the ambulacral plates differs according to position 
and age. They are high near the peristome in young forms, and 
low close to the ambitus ; and they are less unequal in adults. 
The horizontal sutures between the plates are often slightly 
furrowed. 
The plates may be primaries, or compound with two or three 
constituents. The plates near the apical system and extend- 
ing towards the ambitus are primaries. Taking the specimens 
in the British Museum, Nos. 38742, 723, and E ISO, and also the 
half-grown specimen in our possession as examples, it is to be 
observed that near the radial plates there are low r , broad primary 
plates, each having a pair of pores placed close to the interradial 
suture*. The pores are large and separated by a ridge; and 
* The late Dr. Wright (op. cit. p. 207) seemed to separate the poriferous 
zones from the ambulacral plates ; for he wrote, “ The poriferous zones are 
