15° 



ECHINOIDEA. I. 



14. Echinus affinis n. sp. 



PI. V. Figs. 4, 3. PI. XV. Figs. 3, 10. PI. XVI. Figs. 6, 20. PI. XVIII. Figs. 4, 16, 28. PI. XIX. Fig. 27. PI. XX. Figs. 17, 21. 

 This species resembles much Ech. Alexandria together with which it is often found; a closer 

 examination shows, however, that several good characters are found distinguishing it from this species. 

 The test is generally evenly rounded on the abactinal side, but it may be almost as flattened as in 

 Alexandri. The actinal side is generally less flat than in the latter species; the edge somewhat curved 

 inwardly; the peristome rather large. 



All the measures in millimetres. 



The ambulacral areas (PI. XV Fig. 10. PI. XVI. Fig. 20) generally half as broad as the inter- 

 ambulacral ones; at the edge of the mouth they are of equal breadth. There are 73 — */z time as many 

 ambulacral as interambulacral plates; in proportion to the size the number is a little larger than in 

 . llexandri. The arcs of pores are rather erect, they do not always reach quite to the edge of the area. 

 The boundaries between the small plates rather indistinct, the boundary line between the areas somewhat 

 sinuate. A primary tubercle is found on all the ambulacral plates, but they show a very peculiar 

 arrangement. In some specimens the two series are about equally strong, but then they are both very 

 irregular, large and small primary tubercles occurring among each other without any order (PI. XV. 

 Fig. 10). It reminds much of Ech. uorvegicus (PI. XV. Fig. 16); but in the latter the principal series is 

 formed by both primary and secondary tubercles, while in Ech. affinis it is formed by primary 

 tubercles alone, as is sufficiently shown by their position. In other specimens the tubercles decrease 

 evenly in size towards the peristome and the apical area, but then the two series of the same area 

 are of very different size (PI. XVI. Fig. 20). The largest primary tubercles are not much smaller than 

 those of the interambulacral areas. The secondary tubercles are very few and almost only found on 

 the actinal side; they are small and form no longitudinal series. 



The interambulacral areas (PI. XV Fig. 3). The primary tubercles are large and strong; they 

 decrease almost not at all in size towards the apical area, but in the common way down towards the 

 mouth. The scrobicular areas scarcely touch each other on the actinal side, on the abactinal side the 

 distance between them is considerable, the plates being rather high. The secondary tubercles are very 

 few on the abactinal side, which has a similar naked appearance as in Ech. Alexandri. On the 

 actinal side they are numerous, but form no regular longitudinal series inside or outside of the prim- 

 ary series, and they are far from equalling the primary tubercles as to size. The miliary tubercles are 

 generally little numerous; in some specimens, however, they may be so conspicuous as to deprive 

 the test of its smooth appearance. 



