428 Prof. S. Loven on the Structure of the Echinoidea. 



two-pored plate. While tlie peristome is still pentagonal, and 

 the mouth only, a very little removed from the middle of the 

 buccal membrane, a sphseridium has appeared in the two-pored 

 peristomial plate also in Echinocardium ovatum ; but this is still 

 much smaller than the first. Thus, even here, series I. a-Y. h 

 comes after series I. J-V. a. Wlien the mouth has shifted its 

 position so far backwards that it nearly touches the labrum, 

 each ambulacrum shows a third sphaeridium in plate 2 in series 

 I. h-Y. a ] and when the mouth begins to be concealed by the 



frojecting labrum, the fourth is added in plate 2 in series 

 . a—Y. h, apparently first in the paired rows of the trivium. 

 A very young Cassidulus carihcearum^ 4 millims. in length, has 

 already two spharidia, the unequal size of which shows that 

 of the one-pored peristomial plate to be the older one ; when 

 7 millims. long it has four, all visible, in deep cavities. When 

 it has reached a length of 12 millims., there are six sphseridia, 

 but the overgrowth characteristic of this group has likewise 

 commenced. An outer layer of shell-substance of irregularly 

 reticulate texture, closely resembling a froth, spreads upon the 

 surface of the test ; it is seen most distinctly in the sternal region 

 of the unpaired interradium, where it starts from the middle of 

 each plate and extends over its margins in narrow, tortuous, 

 irregular ridges, so as finally, in the adult, to form the uniform 

 surface witli small scattered holes which is peculiar to these 

 genera. In the ambulacra this excrescent layer is first seen to 

 raise the margins of the depressions in which the splueridia 

 are seated, thus forming a projection from them which gradually 

 coversthe splijcridia; and just as of each pair the first spharidium 

 makes its appearance in series I. b-Y. a, so it is also this which 

 in its order is first concealed, and afterwards that of series 

 I. a-Y. b, so that they are overgrown in the order of their 

 appearance. 



Toxopneustes drobachensis again may show how the sphgeridla 

 appear in the Latistelte (see PI. XIV. figs. 1-8). An indi- 

 vidual of 3 millims. (fig. 2) has two sph^eridia in each ambula- 

 crum : — one, the oldest, on the binary plate of the first primary 

 plate of series I. b-Y. a, near its margin ; another on the ternary 

 plate of series I. a~Y. b. At the size of 6 millims. (fig. 3) their 

 number is four ; 1 stands still nearer the margin of its primary 

 plate 1, and 2 as before, and two new ones have made their 

 appearance — 3 on the other primary plate of the binary large 

 plate of series I. b-Y. a, and 4 on the third primary plate (3) 

 of the ternary of series I. a-Y. b, which is an entire plate, not 

 on 2, which is intermediate and a half plate. When the animal 

 is a little more grown (fig. 4), it has also four sphffiridia, not, 

 however, 1, 2, 3, 4, but 2, 3, 4, 5. The binary large plate 



