6o 



ECHINOIDEA. II. 



have constantly found both the inner plates of I and V developed; but the plates I. b. i and Y. a. i 

 are generally very small and easily overlooked. The plates I. a. i and V. b. i may be very unequally 

 developed, one of them simulating the labruni, but the presence of the pore at its inner end shows 

 its real nature. Generally only the four larger of these plates bear distinct pores and tube-feet, in the 

 other plates only quite rudimentary pores are present, sometimes the pore has even quite disappeared. 

 Resides the supposed coalescence between the two inner plates of ambulacra I and V, 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, IV. a, V. b are the 

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

 the rule, only as to ambulacrum IV I have sometimes been unable to see it distinctly. As it seems very 

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



must venture to suggest that Loven 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 ambulacra I and II on one side and IV and 

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

 as the rudimentary inner plate of the interambulaera 1 and 4. Whether this 

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

 it follows from its occasional (not very seldom) occurrence that the plates 

 interpreted by Loven as No. 1 of the interambulaera 1 and 4 (On Pourta- 

 lesia. PL 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 

 Pourt. phiale). 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 hereby that the mutual 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. PL VIII show some of the variations in the structure of this region found in 

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

 in specimens from the same station). 



The primary tubercles form distinct longitudinal (from a morphological point of view: trans- 

 verse) series on the sides at the anterior end of the test. These series generally are very prominent 

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

 one series in the middle, the tubercles increasing 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 thev are upon the whole quite 

 irregularly arranged, though sometimes there is a tendency towards a serial arrangement. The plates 



Fig. 9. Actinal plastron of 



Pourtalesia Jeffreys/. After 



Loven. 



