156 



ECHINOIDEA. I. 



primary tubercles form a more regular series, the secondary tubercles being here considerably smaller 

 than the primary ones, so that they here only to a smaller degree or not at all contribute to the for- 

 mation of the series of tubercles, and here often more plates in succession have a primary tubercle. 

 The pores reach quite to the edge of the area. 



The primary tubercles of the interambulacral areas (PI. XV. Fig. 2) form a very conspicuous 

 longitudinal series, in large specimens sometimes with a few interruptions near the apical area. The 

 secondary tubercles are very few and small on the abactinal side; on the actinal side, as usual, they 

 are more numerous, and some of them become almost as large as the primary tubercles. In larger 

 specimens they often, but not always, form a longitudinal series inside of the primary one; generally 

 they are much larger in one series of plates than in the other. Those outside of the primary series 

 are, as usual, smaller, but more numerous; in smaller specimens they are generally arranged in a 

 rather distinct longitudinal series, in larger specimens most frequently irregularly placed. 



The spines are on the abactinal side rather few; they are long and pointed, especially in small 

 specimens, the interambulacral ones considerably longer than the ambulacral ones, corresponding to 

 the mutual relation of the sizes of the tubercles. On the actinal side they are, as usual, more close-set, 

 and, as in the two other forms, they are flat and widened at the point. The primary spines on the 

 abactinal side decrease only little in length towards the apical area; on the actinal side they decrease 

 very much in length towards the peristome; they do not, however, here form so fine, even an ambu- 

 latory surface as in the two other forms. 



The colour of the test is in small specimens often very characteristic (PI. I. Fig. S. PI. II. Fig. 6). 

 There are 5 large, red spots on the interambulacral areas, and 5 narrow ones on the ambulacral areas, 

 the boundaries between the areas are white. The spots reach to the ambitus, the actinal side is white. 

 On the apical area there is most frequently a rather regular, white pentagon whose corners are formed 

 by the ocular plates; thus the genital plates are white in the inner part, red in the outer part (with 

 the genital pore). The periproct generally slightly reddish (this coloration of the apical area occurs 

 also often in var. Flemingii). In larger specimens (PL I. Fig. 4) the red spots often spread over the 

 whole abactinal side and some way down on the actinal side. The spines (PI. II. Fig. 2) are generally 

 red or reddish brown on a larger or smaller part at the base; this colour passes evenly into a 

 greenish, at last slightly yellowish green colour. Often the spines are red in their whole length, 

 especially the ambulacral ones. On the actinal side the spines are more whitish or quite white; in 

 small specimens (PI. II. Fig. 6) the spines are only slightly coloured. 



Beyond the features described here scarcely any character of greater importance for the distin- 

 guishing of the three forms can be mentioned. Therefore I shall treat the other features together. 



The apical area (PI. XVI. Figs. 5, 10) without marked peculiarities; in larger specimens rather 

 numerous tubercles are most frequently found, arranged circularly along the inner edge of the genital 

 plates. The periproct covered by numerous small plates the largest of which carry a small tubercle. 



The size of the peristome is very varying, especially in var. norvegicus. The buccal membrane 

 is smooth, but contains rather numerous simple fenestrated plates among which more or fewer 

 bihamate spicules may be found; the plates inside of the buccal plates are smaller, a little more 



