8 



secondary tubercles are rather numerous, small, nol disposed in distinct series. 

 Miliary tubercles not very numerous. The median part of the interambulacra is 

 naked on the upper 3—4 plates. The pits are large, rectangular, deep, leaving 

 only a narrow band free in the middle of the horizontal suture, as broad only as 

 the base of the primary tubercle; they are distinct also on the lower side. 



Along the edge ol the pits in both areas there is a rather broad membra- 

 nous brim, leaving only a small oval hole in the middle (Fig. 8). Duncan (On the 



genus Pleurechinus, its classifactory position and alliances. 

 Journ. Linn. Soc. Zoology. XVI. 1882. p. 453) has evidently 

 seen the same structure in Pleurech. hothryoides. It seems 

 probable that these large pits may have some special 

 function. I have made sections of the test in order to see 

 if there be any special histological structures indicating 

 sensory organs or the like, but failed to see anything of 

 Fig. 8. Part of interambula- that kind. The preservation of my specimens, however, is 

 cral area of P/<'iim7ii;iiis sin- not good enough for settling the question. There is no- 

 meiisis: .showing the mem- thing to support the suggestion that thev may have some- 

 l>rane covering tiie pits. (Seih. j • ^^ ^^ ^j^j^ jj^^ hatching of the eggs and young ones 



(Duncan, p. 453). It might seem reasonable to ascribe to 

 these structures a respiratory function; but observations on the living specimens 

 and histological researches on specimens preserved for that purpose are needed for 

 settling the question. 



The apical area (PI. II. Figs. 14 — 15) is rather large, 28 mm. to a diameter 

 of test of 75 mm. The depression at the inner end of the rather large ocular plates 

 may be large and almost tripartite or quite indistinct. There is a distinct secon- 

 dary tubercle on each ocular plate; none of the plates reaches the periproct. The 

 form of the ocular plates is peculiar, the part outside the pore looking like a small, 

 separate, white plate. The ocular pore cannot be seen from above, as is generally 

 the case in the P/eurec/i/nus-species. The genital plates have 1 -3 tubercles at the 

 inner edge; the outer part is mostly dark coloured. Genital openings rather large. 

 The periproct is covered by one large anal plate and a few smaller plates; the 

 anal opening is excentric. 



The buccal membrane is naked, with rather numerous, very slender, but 

 rather large bihamate spicules (PI. VI. Fig. 16). Along the inner edge around the 

 mouth there is a single series of small irregular plates. The buccal plates are 

 small, round, colourless (not green as in Döderleini); they are paired, and all 10 

 buccal tube feet are developed as is normally the case in Echinids. The gills con- 

 tain the usual irregular plates. The auriculæ are rather large and connected. 



The globiferous pedicellariæ (PI. VI. Fig. 36. PI. VII. Fig. 14) have no distinct 

 lateral teeth, only a rounded knob on the one side; the blade is of medium length, 

 the basal part evenly rounded. The ophicephalous pedicellariæ (PI. VII. Fig. 44) 



