10 TH. MORTENSEN, ECHINOIDEA. 



Test regularly arched on the aboral side, flattened on the oral side. 



Ambulacra with a primary tubercle on each plate, forming a regular series close 

 along the pore area. Inside the primary tubercle another tubercle appears from about 

 the 10th — 12th plate from above, at first irregularly, not on every plate, and at the 

 median edge of the plate. Farther downwards this tubercle approaches the middle of 

 the plate forming a more regular series, and towards the ambitus there is thus formed an 

 inner series of tubercles, nearly as regular as that formed by the primary tubercles, and 

 the secondary tubercles are here slightly larger than the primary ones. At the am- 

 bitus again another series of small tubercles appears at the median edge of the 

 plates, forming a short irregular median series; a few of them nearly reach the size 

 of the primary tubercles. Some few miliary tubercles are found, mainly along the 

 upper edge of the plates. The pore zone is rather broad, naked or, below the am- 

 bitus, with a few miliary tubercles. The triplets of pores are oblique, straight. The 

 sutural grooves are well defined, broad, reaching from the median line to the base 

 of the primary tubercle. Towards the ambitus, by and by as the secondary tubercles 

 grow larger, the groove is narrowed by them and at the ambitus so much so, that the 

 edges of the groove join and the groove is thus divided in two, a larger part from 

 the median line to the secondary series of tubercles and a small part between the 

 secondary and the primary series. Below the ambitus this outer part of the groove 

 disappears. At the median end the groove suddenly deepens into a deep distinctlj^ 

 limited hole. 



Interambulacra. The tubercles form a distinct transverse series of four, some- 

 times five, at the ambitus, the number gradually diminishing towards the peristome 

 and towards the apical system. The primary tubercles do not exceed the other 

 tubercles in size, except in the proximal and distal part of the area. Some few 

 miliary tubercles occur at the distal edge of the plates, especially one to each side 

 of the primary tubercle; a slight elevation proceeds from their base towards the 

 primary tubercle, which gives some impression of a radiation from the latter. The 

 crenulation of the tubercles, even the smaller ones, in both areas is fine, but distinct. 



The grooves of the interambulacra are well defined, very broad, the inner ones 

 reaching from the median edge, where they terminate in a small, but sharply defined, 

 deep cut, to the base of the primary tubercle, and the outer ones, in the same way 

 reach from the outer edge of the area to the base of the primary tubercle, the two 

 grooves being separated here only by a very narrow bridge. The secondary tubercles 

 tend to narrow the grooves, and just below the ambitus this narrowing goes so far 

 that the edges actually join, and the groove is divided, the outer in two, the inner 

 in three j)arts, so that we have here a series of .5 small, distinct grooves along each 

 horizontal sviture. Nearer the peristome the intermediate grooves gradually disappear, 

 only the inner and the outer groove remaining, and on the 2 — 3 plates nearest the 

 peristome also the inner pore has disappeared. Along the border between the ambu- 

 lacral and interambulacral plates there is a series of quite small pores, four to each 

 interambulacral plate, situated in the angles of the components of the ambulacral 

 plates. 



