84 SVEN LOVEN, ON THE ECHIXOIDEA DESCKIBED BY LINN^EUS. 



gular rows parallel to tlie long axis of tlie plate, whicli below 

 are directed obliquely iipwards so as reciprocally to converge 

 angularly towards the middle sutiirc, until nearer the top they 

 gradually assume an almost parallel direction. Of the vertical 

 series the marginal ones on each side are primordial, normally 

 regular and continiious all through, very rarely interrupted, 

 and nearly convergent superiorly. Their tubercles are not 

 always the largest. They are followed inwards by other suc- 

 cessive primary vertical series, in adult specimens from two 

 to five, gradually abbreviated below and above. This dispo- 

 sition is attained during growth and by slow and unequal 

 degrees, the process of the formation of spines and tubercles 

 beino- here, as in Echinoids oenerallv, not a concomitant of 

 that of the plates, but independent of it. In a young spe- 

 cimen of 5 mm., of the well-known A. ccquituberculata whicli 

 is to have on either side six vertical series, there are only 

 two series. Of these the first on either side consists of seven 

 tubercles of whicli the uppermost is very small, and the last- 

 formed plate is still without any. The second vertical series 

 has its uppermost very minute tubercle as far back as on the 

 fourth plate from the top. Thus it comes that, owing to the 

 retarded appearance of the tubercles forming the succeeding 

 series, the disks of the interradia in not fullgrown specimens 

 are more or less bare and present au indication of a »står», as 

 Blainville called it. In the species of the Atlantic Ocean, 

 and in one alone of the \Yest iVmerican, this står disappears 

 entirely on the adult; within the other group, of species chiefly 

 from the Pacific side, it is otherwise. In the Arbacia alter- 

 naus, Dufresnei, spathuligera, stellata, punctulata, the formation 

 of successive tubercles on the disks is not merely retarded, 

 but nearlv altooether arrested, and accordinoly the står be- 

 comes in the adult strikingly manifest. 



As a rule each additional vertical series of tubercles makes 

 its appearance near the middle suture of the interradium. 

 The plate lengthens transversely and a new tubercle is formed 

 on its inner part. As in all the Echinids the spines and their 

 tubercles begin to form on those plates Avhich during growth 

 become peristomal, and procced upwards, growing successively 

 more slowly above the ambitus. At the ambitus, where the 

 test is widening at the greatest råte, the latest tubercles be- 

 come comprised within a small, more or less distinct, lanceolate 



