50 



PKOF. P. M. DUNCAN AND MR. W. P. SLADEN ON THE 



basal. This is erroneous, and so is the figure given in his plate. 

 The same specimen shows the very small size, comparatively, of 

 the radial plates (oculars). 



It is evident then that the adult forms of Discoidea cylindrica 

 have all the five basals perforated for the madreporite, and all, 

 except the fifth, perforated for the genital duct. 



The Amhulacra. — "Wright was the first to point out that the 

 simple, straight condition of the pairs of pores was not univer- 

 sally fiDund in the species, but that the pairs on the actinal area 

 might fall into a biserial arrangement. 



Now it aj^pears 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 diff'ers 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 tlie 

 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 diff'ers 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 180, and also the 

 half-grown specimen in our possession as examples, it is to be 

 observed that near the radial plates there are low, 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 be wrote, "The poriferous zones are 



