6o 



ECHINOIDEA. II. 



have constantlv found both the inner plates of I and V developed; but the plates I. b. i and \'.a..i 

 are o'enerally verv small and easily overlooked. The plates I. a. i and \'. b. i nia\' be very unequalh' 

 developed, one of them sinuilating the labrum, but the presence of the pore at its inner end shows 

 its real nature. Generalh- onh' the four larger of these plates bear distinct pores and tube-feet, in the 

 other plates onl\- quite rudimentar\- pores are present, sometimes the pore has even quite disappeared. 

 Besides the supposed coalescence between the two inner plates of ambulacra I and \'. Loven, points 

 out (Op. cit. p. 36) as another peculiar feature in this species, that the inner plates of ambulacra II and 

 IV are not in accordance with the general rule that the plates I. a, II. a. III. b, I\'. a, V. b are the 

 largest. I have constantl\' found the inner plates of the paired ambulacra to be in accordance with 

 the rule, only as to ambulacrum l\ I have sometimes been unable to see it distincth-. As it seems very 

 unlikely that all the specimens examined by Loven should happen to be abnormal in this respect, I 



must \enture to suggest that Lo\'en has overlooked some of these small 

 plates, which may, indeed, be rather difficult to see. (I have found them 

 easiest to discern when examining the denuded test in alcohol; on dried 

 tests, treated with alcohol-glycerine it is almost impossible to trace the limits 

 between the small plates). A very small plate may sometimes be found be- 

 tween the inner plates of the ambidacra I and II on one side and IV and 

 \' on the other side (PI. VIII. Figs. 5, 8, 9, 11). It must doubtless be regarded 

 as the rudimentary inner plate of the interambulacra i and 4. Whether this 

 plate was really absent in Loven's specimens or perhaps was overlooked, 

 it follows from its occasional |not ver}- seldom) occurrence that the plates 

 interpreted by Loven as No. i of the interambulacra i and 4 (On Pourta- 

 lesia. PI. II. 9) are really No. 2. In the figure 9 copied from the quoted figure 

 of Loven, I have shown my interpretation of these plates. (Comp. Figs. 10, 11 of 

 Poiirt.pliiiilr). Upon the whole there is so great variation in the development 

 of the plates of this region that it is scarcely possible to find two specimens 

 quite alike in this respect. Such extensive variation in structures of consider- 

 able morphological importance is of no small interest, and it is shown herel)\- that the nnitual relation 

 of the plates in this region cannot be relied upon for specific differences, in any case for this species, 

 and for the other species it will also be necessary to be very cautious in the use of such characters. 

 The figures 4—6, 8—11. PI. \'I1I show .some of the variations in the structure of this region liiunil in 

 P- Jeffreys/. (These specimens otherwise are all quite typical P. Jeffreysi; all variations may be fouml 

 in specimens from the same station). 



The primary tubercles form distinct longitudinal (from a nicirplu)l(igical \w\\\\ of view: trans- 

 verse) series on the sides at the anterior end of the test. Tliese .series generalh- are very prominent 

 on the plates of the anterior series of the two antero-lateral ambulacra (II and 1\'), each plate bearing 

 one series in the middle, the tubercles increa.sing somewhat in size from the anterior towards the 

 posterior edge of the plate. On the plates of the posterior series of these two ambulacra the tubercles 

 are more irregularly arranged, and on the posterior part of the test they are upon the w lu)le quite 

 irregularlv arranged, thougli sometimes there is a tendency towards a serial arrangement. The plates 



Fig. 9. Actinal plastron of 



Pourlalesia Jeffreysi. .\fter 



I.oven. 



