THE FAMILY AEBACIAD^. 51 



or arcs is in reality a secondary formation caused by the move- 

 ment of the peripodia and the modification of the plates above 

 noticed. It may be conjectured that the two more adoral pori- 

 ferous plates of the first compound ambulacral plate have been re- 

 sorbed or merged in the peristomial rim, and that the peripodium 

 of the aboral demi-plate alone remains, its adoral foramen notching 

 the peristome. The peripodium of the adoral demi-plate of the 

 second ambulacral plate is also very near the margin, and may 

 be said to partially notch it. The growth stages upon which 

 this merging and obliteration of plates depend have been care- 

 fully described by Loven {op. cit.) in the case of Strongylocen- 

 trotus drcebacliiensis, and need not be recapitulated here. 



In A. stellata the same general arrangement and relative pro- 

 portions of the poriferous plates in the ambulacral compound 

 as those above described occur. The outline, however, of the 

 plates is different (see fig. 5). It will be observed that the 

 small ad- and aboral poriferous plates — the demi-plates— of the 

 triplet have a straight or truncate inner end, parallel to the 

 outer line of the ambulacral area, the suture-line mounting the 

 base of the boss at a right angle to the ad- and aboral margins 

 of the compound ambulacral plate respectively, and is then 

 sharply bent, forming an angle less than a right angle, to direct 

 its course towards the outer margin of the ambulacral area. 

 The outline of the demi-plates is thus more or less trapezoid, and 

 their greatest depth is at their inner end. In consequence of 

 this the intermediate portion of the main primary poriferous 

 plate is somewhat cuneiform, expanding as the margin of the 

 ambulacral area is approached, where its greatest depth is 

 situated. The same narrowing of the plates in the neighbour- 

 hood of the peristome takes place as already noticed in A. pus- 

 tulosa, and the peripodia in consequence become crowded (sea 

 fig. 4). Their arrangement may be readily formulated by com- 

 paring them with fig. 5. 



In A. punctulata the ambulacral structure approaches in 

 character very closely to that of A. stellata, as reference to fig. 2 

 will suffice to show. 



The manner in which the ternary compound ambulacral plates, 

 with which we are now concerned, are formed, may be well 

 studied on the upper portions of the ambulacral areas of any of 

 the species of Arhacia above mentioned ; and their development 

 is highly instructive. In the neighbourhood of the apical disk 



