the Morphology and Classification of the Saleniidte. 125 



plate, and the adoral peripodiura, h, is close to the iiiterradium. 

 The lowest peripodium of the apparent triplet, c, covers nearly 

 the whole of its plate, which is crowded out from and does not 

 reach the median ambulacral suture ; it does not form part of 

 a compound plate, and it is a demi-plate (single). The 

 crowding of the plates at the very edge of the peristome is 

 still greater and the sutures are obliterated in the specimens ; 

 but it is evident that there are derai-plates there. In a speci- 

 men in the British Museum (fig. 4) there is a triple com- 



Ambulacral plates of AcrosalenicB ; much of the ornamentation 

 is omitted. 



pound plate between the ambitus and the position of the 

 commencing crowding of the pairs, in which pressure has united 

 the usual two primaries, a and h, and also an adorallj placed 

 primary, c. The cause relates to the growth of a tubercle, 

 and the adoral primary (c) has been so pressed from above 

 downwards that it barely reaches the median ambulacral 

 suture and is almost a demi-plate. This is the first step to 

 the more decided crowding and deformation which are seen 

 nearer the peristome. 



