124 Prof. P. M. Duncan and Mr. W. P. Sladen on 



believed to be low primaries, each with a pair of pores ; it 

 has also been stated that in neither of the types is there any 

 biserial arrangement of the pairs near to the peristome. But 

 the crowding out of pairs and the biserial arrangement has 

 been described in every species of Acrosalenia. The con- 

 struction of the ambulacral plates has, however, not hitherto 

 been described, and it is very interesting and important in 

 classification. 



Desor is only partly correct in his statement that the pairs 

 of pores in Acrosalenia are " simple," meaning that there was 

 a pair for each plate and that there were no compound plates. 

 The ambulacral plates are simple for varying distances from 

 the radial plates, and are long low primaries, each of course 

 with a pair of pores, the peripodium being well developed and 

 the nodule between the pores of a pair often being broad and 

 high. 



In Acrosalenia sjyinosa the straight vertical row of pairs is 

 interfered with not far above the ambitus, and three or four 

 pairs of pores begin to be in slight curves, and this condition in- 

 creases with the dimensions of the ambulacral tubercles. It is 

 easy to trace, in weathered specimens, that there are compound 

 plates in that part of the ambulacrum, and extending thence 

 to the peristome. The plates have been originally primaries, 

 and have been compressed from above downwards and in the 

 contrary direction by the succession of plates and the growth 

 of the comparatively large ambulacral tubercles. The succes- 

 sion is a simple primary followed by a compound plate made 

 up of two primaries formed into a geometrical plate, tlie upper 

 one, a, being the smaller and having its adoral sutural line 

 passing from the adoral pore obliquely upwards and towards 

 the median ambulacral suture, and crossing the tubercle on its 

 aboral shoulder (fig. 1). The other or adoral primary, b, is 

 larger, especially at the median suture of the ambulacrum, 

 the greater part of which is formed by it. The two primaries 

 constitute a compound plate and are formed into a geometrical 

 fio-ure. Then comes a single primary without a large tubercle, 

 and this plate is broad, low, and about one half of the height 

 of the compound plate above it. A compound plate comes 

 next, and it closely resembles the one above ; but the suture 

 may be more transverse, and it is followed in its turn by a 

 simple primary (fig. 2). This succession is repeated several 

 times until close to the peristome, where crowding and decided 

 curvino- in arcs of the peripodia occur (fig. 8). But the 

 crowdino- (fig. 8) is not after the type of the Triplechinidge, 

 for instead of there being compound plates with three com- 

 ponents, only two primaries (a and b) occur in a compound 



