74 



ECHINOIDEA. II. 



pushed far backwards, separated from the small inner plate at the peristome not only by the ambu- 

 lacral plates IV. b. 24 and II. a. 2 3, the posterior one of them being much prolonged backwards 



along the ambnlacral plates V.a.i and 

 I. b. i, but also by the interambulacral 

 plates 4. a. 4 and i.b-4 5, which join 

 the ambulacral plates V. a. 1 2 and 

 I. b. 12 for a long way. The plates 

 i. a. 4 and 4. b. 4 are very much en- 

 larged, and upon the whole all the 

 plates of these interambulacra are 

 unusually large. As in piiiale these 

 interambulacra are very much bent 

 upon themselves, the median part 

 being near the posterior end, where- 

 as the upper and lower end is at 

 the anterior end of the animal. That 

 the interpretation of the plates given 

 here is correct seems beyond doubt, 

 from a comparison with P. phialc 

 (Fig. 10), in which the interpretation 

 lies quite evident. The plates of 

 the antero-lateral ambulacra and in- 

 terambulacra are rather small, in 

 accordance with the small size of 

 the head. The odd anterior am- 

 bulacrum contains ca. 14 pairs of 

 plates ; I have been imable to count 

 the number with full certainty. The 

 invagination is comparatively small, 

 but otherwise of the usual form. 

 The peristome is almost round, cov- 

 ered with rather large plates. The 

 mouth is a little below the middle. 

 The apical system is situa- 

 ted near the anterior end; from the 

 outside I was unable to see the 

 limits of the plates in this region 

 with any certainty, but from the inside most of them could be distinctly seen (Fig. 14). Probably the 

 plate just behind the large inner prolongation from the madreporic plate will really be divided in 

 two, but I could not distinguish any line there. Also the small innermost plate of the antero-lateral 



Fig. 13. Analysis of part of the test of Pourtalesia paradoxa. 



