Prof. S. Lov^n on the Structure of the Echinoidea. 295 



round its lower aperture, after the upper one becomes filled up 

 and blind. These two pores (1 and 2) form persistently a 

 pair of themselves. The second primary plate (1, 2) is a half 

 plate ; its pore (3) removes very slightly from its original place, 

 and commences the first distinct secondary arc, but, in conse- 

 quence of the diminution and depression of the first primary plate, 

 approaches the margin, where it also in its tm-n loses a part of 

 its wall. The third and last primary plate (1, 3) of the first 

 large plate is again an entire plate ; and its pore (4) also moves 

 inwards, and further than the preceding one. Pore 5, which 

 belongs to the first primary plate of the second large plate, is, 

 as such, again the most movable, wanders far from the suture, 

 and completes the first secondary arc of the three pores 3, 4, 5. 

 Pore 6, situated in the intermediate half plate 2, 2^ remains 

 in its place as the first of the second arc, again of three pores, 

 of which the second, (7), in primary plate 2, 3, has moved 

 inwards not inconsiderably, although not so much as the third 

 (pore 8), which belongs to primary plate 3, 1. With pore 9, in 

 primary plate 3, 2, again commences a third secondary arc of 

 four pores, which move in accordance with the same law as 

 the preceding ones — namely, 9, pi. 3, 2, 10, pi. 3, 3, 11, pi. 3, 4, 

 and 12, pi. 4, 1. Whilst these movements have been going on, 

 the large plates 1, 2, and 3 have also become more strongly com- 

 pressed. In one individual (fig. 2) they constitute two thirds 

 of the whole height of the corona, and the greatest periphery 

 nearly coincides with the sutm-e between 2 and 3 5 in fig. 3 they 

 all lie below the line of the greatest periphery, occupy less than 

 half the height of the corona, and their width is rather greater 

 than their height. The first large plate (1), the peristomial 

 plate, especially, is strongly compressed ; its pores cease to 

 grow ; their upper tube is diminished or closed ; pore 2 loses 

 still more of its wall in the margin of the peristome; pore 3 

 gradually follows in tlie same direction ; the radiolar tubercles 

 disappear entirely or for the most part ; and in the individual, 

 .fig. 6, the suture between the large plates 1 and 2 has dis- 

 appeared, and they have coalesced to form a single binary large 

 plate of the second order, 1 + 2, composed of six primary 

 plates, wliich number cannot be distinguished ; and the large 

 radiolar tubercle it bears is that which originally belonged to 

 large plate 2. In the individual, fig. 7, this double large plate 

 1+2 has become still more compressed; of pore 2 only half 

 remains, and but little more of pore 3. The sutures of the 

 primary plates now disappear still more within the large plate 

 3 ; and in the individual, fig. 8, even this has completely 

 coalesced with 1 + 2 to form a single ternary plate of the 

 third order, 1 + 2 + 3, composed of eleven primary plates, and 



