REPORT ON THE ECHINOIDEA MORTENSEN 



Measurements 



277 



Note.— The larger of the specimens measured is from Challenger station 204, the smaller from Alba 

 tross station 5415. 



Description. — The test is flattened above and below, with the sides 

 beautifully arched; the circumference is circular. The ambulacra are 

 very little sinuate. The interporiferous zone is about twice the width 

 of a pore zone. The marginal series of tubercles is very regular, the 

 tubercles being small and not contiguous. Within the marginal tuber- 

 cle each plate carries a number of small tubercles which are not 

 regularly arranged, covering the whole plate and leaving only the 

 horizontal edges free, which are sunk so as to form distinct grooves, 

 narrowly connected through the likewise sunken vertical suture. 

 The whole interporiferous zone is otherwise flat, scarcely at all slop- 

 ing toward the midline of the area. The pores are rather small, 

 equal sized, and rather distant, being separated by a fairly broad 

 wall which rises into a low rounded tubercle. The ridge separating 

 the adjoining pore pairs is low. The whole pore zone is scarcely at 

 all sunken (pi. 74, figs. 1-2). 



In the interambulacra the areoles are rather low, the subambital 

 slightly transverse-oval; the three or four proximal ones are confluent. 

 The horizontal suture between the uppermost plates is more or less 

 deepened. The scrobicular ring is inconspicuous, with the tubercles 

 not much larger than the marginal ambulacral tubercles. The median 

 part of the plates is closely covered with small tubercles which are 

 rather distinctly arranged in horizontal series. The inner end of the 

 horizontal sutures are deepened so as to form a fairly conspicuous 

 groove; the adradial end of the sutures also shows an indication of a 

 groove. The vertical median suture is narrowly naked and slightly 

 sunken, the whole median interambulacral area sloping gently toward 

 the median line. The width of the median area is about two-thirds of 

 an areole. 



The apical system is from 50 to 55 per cent of the horizontal diame- 

 ter, slightly raised in the middle, but not thickened so as to rise with 

 its edges above the level of the test as is the case in some other gonio- 

 cidarids, for instance, Discocidaris mikado. The oculars are widely 

 exsert in the Albatross specimen, while in the specimen from Challenger 



