234 BULLETIN OF THE 



in structure to those of Podoeidaris). The poriferous zone has in the 

 earliest stages the structure found in the adult, only it does not widen 

 at the aetinostome. The ratio of the actinostome to test doe9 not vary 

 greatly in different stages of youth ; the edge of the actinal system form- 

 ing the groove of the gills is turned back but slightly in young, the lips 

 taking the place of cuts becoming more prominent (Boletia-like) with in- 

 creasing age. The separation of Echinocidaris and Arbacia to represent 

 the groups with bare or crowded interambulacra is not natural, depend- 

 ing upon the greater or less resorption of the rudimentary tubercles 

 formed in the earlier stages. It is very common to find young of 

 Echinocidaris punctulata which would pass for young of Arbacia, and 

 young Arbacia requituberculata which would pass for young Echin- 

 ocidaris. Owing to the independent growth of the plates of the porif- 

 erous zone, we have either three or four pairs of pores for each am- 

 bulacral plate ; the same is the case with other Oligoporidae, as limited 

 by Desor, showing that the division he has made, convenient though it 

 is as a key for the easier grouping of genera, is yet not strictly reliable, 

 the mode of growth of many Polyporida) showing in their young stages 

 that they have but a small number of pores (Tripneustes, Mespilia) for 

 each ambulacral plate which places them among the Oligoporidae; but, 

 owing to the independent growth of the plates of the poriferous zone in 

 older stages, they seem to belong to the Polyporidie. 



In Echinus, Sphaerechinus, Lytechinus, we find in the younger stages 

 the same unbroken vertical arrangement of the pores, taking next a 

 vertically arched form, still connected, and then assuming the arrange- 

 ment of the adult. In these genera the anal system is at first covered 

 by one plate, aud undergoes changes similar to those of Toxopneustes, 

 by the addition of four smaller plates, and so on, the original subanal 

 plate retaining long a greater prominence. The miliaries are formed in 

 these genera as well as Toxopneustes by radiating ridges arising from 

 the base of the primary tubercles, forming a sort of star, then they swell 

 at the distal extremity, forming a set of club-shaped spokes round the 

 main tubercle; these are little by little separated from it, and become 

 independent elliptical tubercles at first, and then miliaries or secondary 

 tubercles. The ten large buccal plates of the actinal membrane are the 

 first to appear. Small plates (in genera in which they are found in the 

 adult) are next formed between them and the teeth (Echinus-like), 

 while afterwards they cover the whole membrane, as in Lytechinus 



