60 ROBERT TRACY JACKSON ON ECHINI. 



compound plates tend strongly to fall into arcs in such fashion that succeeding pore-pairs 

 fall in different vertical series, as seen well in Strongylocentrolus franciscanus. In Diplocidaris 

 and Tetracidaris the pore-pairs, though in primary plates, are alternately pulled out of line, 

 so that succeeding tube-feet would not lie quite over one another. 



This passage from the primitive condition of monoserial pore-pairs to the biserial or poly- 

 serial, which is attained by the three methods of more than two columns of simple plates 

 (Palaeozoic), compound plates with pores tending to arcs (Centrechinoida) , or primaries with 

 pore-pairs biserial (certain Cidaridae), is all in accordance with the mechanical principle of each 

 tube-foot attaining fullest play by being out of line with its fellows in so far as is possible. The 

 arrangement reminds one of the phyllotactic arrangement of leaves in plants, where each leaf 

 is not succeeded by another in a vertical line until one or more have been added which are off 

 that vertical line, but in a succession of lines of their own series. 



In types, as spatangoids, in which below the petals the plates are relatively high and tube- 

 feet therefore not crowded, the pore-pairs fall nearly in vertical uniserial lines, as in the young 

 and primitive forms. This is aided probably by the fact that the ambulacral feet are for the 

 most part modified as respiratory rather than locomotive organs. 



Ambulacral plates on their proximal side differ from the same plates on their distal side, 

 and this difference, as far as observed, is usually in the line of greater simplicity and relative 

 primitiveness of structure. In Melonechinus multiporus (Plate 56, figs. 4, 5) and 'M. gigan- 

 teus (Plate 61, figs. 5-9), the ambulacral pore-pairs distally are eccentric and lie nearest to 

 the interambulacral suture, but proximally, as shown, the pore-pairs of isolated plates lie in the 

 middle of each plate, a primitive character. In general the pores proximally are nearer the 

 middle of the plate than on the distal side. In Maccoya intermedia (Plate 34, fig. 2) 

 the plates alternately reach and are cut off from the interambulacral suture and pore-pairs are 

 biserial. All this is as seen from the outer or distal side. When seen from the inner or 

 proximal side (Plate 34, fig. 3), all the plates at the mid-zone cross the half-area instead of 

 alternate plates being cut off from the interambulacral suture. Also the pore-pairs are uni- 

 serial instead of biserial, as they are on the outer side of the very same plates, so that in all 

 these characters they are primitive like the young and the next lower genus Palaeechinus (Plate 

 31, fig. 1). A similar condition is seen in distal and proximal sides of the plates in Maccoya 

 burlingtonensis (Plate 33, figs. 4, 5). In Lovenechinus missouriensis the outer demi-plates, 

 as seen distally (Plate 43, fig. 3) are very narrow, the inner occluded plates are relatively 

 wide and the pore-pairs of both lie very near the interambulacral suture. In a proximal view 

 of the same plates (Plate 43, fig. 4) the demi- and occluded plates are of about the same 

 width, and the pore-pairs of both lie near the middle line of the half-ambulacrum. Also 

 proximally the plate that lies opposite the horizontal suture lines of the adradial plates is spread 

 out? in a fan-shaped manner, whereas on the distal side the fan-shape is nearly or quite wanting. 



