90 PANAMIC DEEP SEA ECHINI. 



plates begins as an independent plate for the whole length of the ambula- 

 cral zone. In Strongylocentrotus the single primary plates are the upper- 

 most of the abactinal plates. 



In this larger specimen (34 mm.) the primary tubercles have increased 

 in number with the increase of the ambulacral plates, and from one to three 

 small miliaries have been added to the outer part of the larger component 

 plate. The buccal plates also have become thickly covered with secondaries 

 and miliaries. A comparison of figs. 2 and .?, Plate 48, will readily show 

 the nature of the changes which have taken place in the tuberculation of 

 the ambulacral system. 



In the larger specimens eight or nine interambulacral plates carry on the 

 actinal side a single, primary tubercle ; it increases in size as it approaches 

 the ambitus. The sixth, seventh, and eighth plates carry two primaries ; the 

 second appears in specimens of about 100 mm. On the abactinal side the 

 primary tubercles become greatly reduced in size, occurring in a single row 

 in small specimens; subsequently there are two and three or four at the 

 ambitus. The scrobicular area of the larger tubercles frequently spreads 

 over the adjoining miliaries, which are thus resorbed (PI. 45, fig. 15). 

 The miliaries covering the test are in all stages of growth. The first 

 appearance of the tubercles is indicated by a concentration of the open 

 reticulation (PI. 46, fig. 2) growing round the areola, which is thus lifted 

 above the general network (PI. 46, figs. 5, 6). 



In the abactinal system of a specimen of 34 mm. (PL 40, fig. 2) very 

 marked changes have taken place. The genital and ocular plates no longer 

 form a closed ring. Four of the genitals are separated from their adjoining 

 oculars by elongate anal plates which have forced their way between them, 

 and connect with the youngest abactinal interambulacral plates of the odd 

 posterior, the left anterior, and the two posterior lateral interambulacra. The 

 odd ocular is not yet separated from its adjoining genitals, and the madre- 

 poric genital only partly so by the intrusion of a wedge-shaped anal plate. 

 Genital pores have developed in all the genital plates except the odd pos- 

 terior one. The tuberculation of the genital and ocular plates is less promi- 

 nent than in the younger specimens (PI. 40, fig. /). On the contrary, many 

 of the larger anal plates carry a single secondary or large miliary tubercle. 

 The madreporic body also covers the greater part of its genital. 



The actinal system measures 12 mm. in diameter and has four rows of 

 buccal plates (PI. 43, fig. 6). They do not fill quite as completely the actinal 



