88 PANAMIC DEEP SEA ECHINI. 



part a vertical line of two or three small secondaries with a single miliary. 

 No trace can be found of the genital pores. 



The ocular plates are larger than the genitals ; from their junction with 

 the genitals their outline forms a re-entering angle along the ambulacra] 

 side of the abactinal interambulacral plates, its sides extending nearly 

 parallel, as far as the middle of the second plate. At their junction with 

 the abactinal ambulacral plates the oculars are cut off at right angles. The 

 ocular pore is placed near the ambulacral edge. The oculars carry a single 

 secondary tubercle in the centre of the anal side, with five to six miliaries 

 in two vertical rows occupying the distal part of the plate. The anal 

 plates cany no miliaries; their number is already so great that, even in 

 the outer row of larger plates, we can no longer trace the original anal 

 plates. Inside of the irregular row of outer plates there are two oilier rows 

 of smaller plates filling irregularly the central part of the anal system. 



In a specimen of 15 mm. in diameter the three buccal plates already 

 show, seen from the interior of the test, the reverse imbrication charac- 

 teristic of the actinal system (PI. 44, fig. /), and the fourth ambulacra! plate 

 shows a small club-shaped prominence, the base of the arch of the auricle. 

 See Fig. Tiii. 



In a specimen measuring 34 mm. in diameter (Id. 30, figs. 3,4) there are 

 five and five interambulacral plates between the primordial plate and the 

 ambitus, and nine and nine between the ambitus and the genital plate 

 (PI. 49, fig. ':.'). The position of the primary interambulacral tubercles has 

 not changed from that observed in the preceding stage described, but with 

 the extension of the interambulacral plates laterally, additional miliaries and 

 small secondaries have formed upon the plates. The left side of the 

 primordial, in addition to its two small secondaries, is well covered with 

 miliaries. On the other plates below the ambitus the miliaries have in- 

 creased in number along the ambulacral edges of the interambulacral plates. 

 A small additional secondary tubercle is found on the inner part of the plate ; 

 the miliaries, however, have not increased in number along the median line. 

 On the plates above the ambitus additional miliaries occur arranged horizon- 

 tally along the middle line of the plate. 



The changes which have taken place in the ambulacral svstem of a speci- 

 men 34 mm. in diameter, compared to one only 10 mm., are much greater 

 than those occurring in the interambulacral zones. We cannot fail to 

 observe the great width already attained by the ambulacral system at the 



