ON THE YOUNG STAGES OF ECHINI. 739 



but a short distance from the apex. The ambulacral and interambulacral 

 plates are of the same size, hexagonal, forming twenty equal zones, carrying 

 but a single large tubercle in the centre of each plate ; seen from below the 

 surface is deeply concave, the mouth much larger in proportion to the test 

 than in adult specimens. We see forming from this side the posterior in- 

 terambulacral lunule as a deep pit (PI. XL. f. 2), at one extremity of which, 

 near the mouth, is placed the anus, about one third the distance from the 

 edge of the test. We find also rudimentary phyllodes made up of a few of the 

 small pores, which eventually extend in the ambulacral furrows to the edge 

 of the test, but are now restricted to a small number clustered round the 

 mouth. The outline in a subsequent stage becomes slightly pentagonal, the 

 plates elongate ; the lunule pierces through to the abactinal side (PL XL. f. 

 3) ; the rosette is also radiating, made up of five to six pairs of pores for each 

 poriferous zone. The ambulacral area is now slightly narrower than the in- 

 terambulacral zones. When the posterior lunule has become a small round 

 opening, encroaching upon the plates of the posterior interambulacral area, 

 extending as a lobe bej'ond the outline of the test, the rosette is slightly 

 petaloid (PI. XL. f. 7). There are from two to five tubercles on each plate ; 

 these are quite elongate, having lost their hexagonal outline ; the lower suf- 

 fice is flat, and on the lower side the ambulacra have broadened very rapidly, 

 the interambulacra forming narrow bands carrying larger tubercles between 

 the ambulacral zones. The edge of the test is still quite thickened, and it is 

 only when the young Mellita has attained somewhat less than half an inch 

 in diameter that the ambulacral lunules appear as pits, seen at first from the 

 lower side only (PL XL. f. m), and gradually forcing their way through the 

 test (PL XL f. 9) to the abactinal side. The posterior interambulacral 

 lunule increases rapidly in size ; the test and the groove in which the anus 

 is placed become somewhat separated from it, being simply a depression in 

 the continuation of the lunule (PI. XL.f. 4. 6, 8-10). After the appearance 

 of the lunules as slight pits, which develop unequally and do not appear 

 simultaneously, the changes are limited to the increase in size of the lunules 

 and of the ambulacral poriferous zone on the lower side ; the outline and 

 general facies (PI. XL.f. 10-12), with the exception of the larger size of the 

 tubercles, being that of the adult, 



The general character of the changes undergone by Echinarachnius and 

 Mellita hexapora, as far as they relate to the transformations of the ambu- 

 lacral rosette, the growth of the tubercles, the changes in the proportions of 



