34 PKOF. p. M. DUNCAN AND MR. W. P. SLADEN ON 



whict. are not great in vertical measurement like those just ' 

 noticed, but low and yet broad. The arrangement of plates 

 differs in the two zones of the ambulacra, as might be expected ; 

 and there is an indefinite alternation of large and small plates in 

 the zones. The large plates are ornamented with small tubercles 

 or with large granules according to the species ; but although 

 the tubercles seem to be on alternate plates first in one zone and 

 then in the other, the examination of the weathered surfaces o£ 

 most specimens shows clearly that there are plates intercalated 

 which destroy this simple and superficial arrangement. It is true 

 that at the ambitus the great plates which carry the tubercles, al- 

 ternate in the zones : but that is evidently because the full develop- 

 ment of the triple-plate combination has occurred. In the region 

 now under consideration, the arrangement in perfect triplets is 

 ouly foreshadowed. Eeally there is a method in the apparent 

 confusion of the interporiferous parts of the ambulacral plates. 



In the compound plates of the parts of the ambulacra already 

 considered, there are two demi-plates and a large |)i'iniary ; and in 

 the radial region primary plates, alone, exist close to the radial 

 plate ; then follow small primaries which are, as it were, kept from 

 growing by the presence of the expanded and future tubercle- 

 bearing parts of other primaries close by, and then a demi-plate 

 is seen here and there. The demi-plate is seen crushed in between 

 two larger plates, and sometimes there are two demi-plates in 

 succession with a large expanded plate close above or below. The 

 poriferous parts of the plates are only exposed to a pressure 

 which is tolerably equal in one direction, and therefore any dif- 

 ference in size relates to symmetrical go-owth. In the iuterpori-- 

 ferous areas the growth is most irregular in consequence of the 

 varying size of the plates ; and the result is irregular pressure and 

 the prevention of any increase in size of some plates, and their 

 crushing and crowding out and away from the median line. As 

 already mentioned, the ambulacral plates near the radial plate 

 and for some distance down, are small primaries, and every one 

 reaches the median line ; but immediately that one of these begins 

 to expand in its interporiferous part, a coi-respondiug want of 

 room is felt by the small primary immediately above, and which 

 is bound to move adorally as others are added above, as well as 

 to endeavour to increase circumferentially. Moviug downwards, 

 not increasing in expansion in a zone which increases in breadth, 

 it follows that the small primary becomes a mere appendage to a 



