46 PEOE, p. M. DUNCAN AND Mil. W. P. SLADEN ON 



In explanation, it is evident that the first three primaries 

 cannot expand ; but their downward growth, together with the 

 expansion of the fifth plate, prevented plate 4 from being any- 

 thing else than a demi. The expansion of plate 5 was not much 

 adorally ; and the consequence was that plate 6 did not suffer 

 pressure enough to form a demi, and it remained a primary ; 

 suffering, however, from direct pressure from above downwards, 

 and being prevented from increasing in a vertical direction, it 

 became a low plate. 



The Peristomial Region of tlie Ambulacra. — The plates aud the 

 tubercles become smaller and closer towards the peristome, and 

 yet, although the plates are lower and narrower from side to 

 side, there is no doubling of the pairs of pores. These are in a 

 series of simple arcs close to the edge of the tubercles, and, ex- 

 cept at the lowest of all the tubercles, the arrangement of the pairs 

 resembles that seen higher up the test. There is a little crowd- 

 ing of the lowest triplet, and the first pair of it may be rather 

 more externally placed than is usual elsewhere. As a rule, the 

 arrangement is rather more simple than that seen in the fossil 

 species. 



Just outside the poriferous zones is a long and narrow plain 

 " tag," and it reaches from the small branchial cut and ends in a 

 point at some distance aborally. The plain construction of this 

 tag is in marked contrast with the arcs of broadly elliptical peri- 

 podia which support the triplets of pairs of pores close by. We 

 do not enter into the consideration of the function of the tag, as 

 it will be treated of by one of us on a future occasion. 



The suturing of the plates of the peristomial triplets is on the 

 same plan as that of the compound plates of the ambitus ; the 

 demi-plates are lower and broader, however, but they are placed 

 in the same positions relatively to the great primary plate as in 

 the ambital regions. 



The auricles are small, discounected, and there is no union by 

 a raised plate crossing the interradia. 



At the peristomial end of the ambulacra, the deep pits for the 

 sphseridia are very prominent objects in the line of the median 

 suture and at those places where it is joined by the transverse 

 sutures. When the ambulacral plates of either side are sepa- 

 rated, so as to divide the sphseridial pits along the median line, 

 the separated faces of the median suture show deep sphaeridial 



