Prof. S. Lov^n on the Structure of the Echirwldea. 381 



lacral plates, which, except the peristomial ones I. a-V. Z*, are 

 always simple and primary, behave in a different manner. As 

 in all the irregular Echinoidea, the corona has a fixed boundary 

 in the peristome. Many young Spatangidas a few millimetres 

 in length, which are more rounded than the older ones, and 

 have the mouth nearer the middle, have the peristome pent- 

 agonal, and so nearly equilateral, that in a still earlier stage 

 it was probably completely so (see woodcut, fig. 3). Its sides 

 all lie in the plane of the test, or are just sunk within it, as 

 in Hemiaster &c. In accordance with this form of the 

 peristome, is the part taken in its formation by the ambulacra 

 and interradia. In opposition to what occurs in Cassidalus^ 

 the former occupy only a small portion, enclosing the angles of 

 the peristome ; the latter, which are much broader, and nearly 

 of equal breadth among themselves, form the greatest parts of 

 its sides. The mouth is now in the middle of the buccal 

 membrane. The considerable change which the peristome 

 subsequently undergoes consists in its ambulacral plates iu- 

 ■creasing in breadth, especially in the trivium, whilst the 

 peristomial plates of the paired interradia do not increase in 

 breadth in the same proportion, especially those of the posterior 

 pair ; and that of the unpaired interradium (the labrum) becomes 

 widened, shoots forth, and arches itself, at the same time that the 

 mouth, having become elongated by degrees, gradually moves 

 backward, so that the greater part of the plated buccal mem- 

 brane soon comes to lie in front of it, and only a narrow border 

 behind it, and this is concealed by the projecting lobes. When 

 the individual is full-grown the ambulacra of the trivium near 

 the peristome are broader than the interradia, with the exception 

 of the labrum ; in Breynia the peristomial plates of the paired 

 interradia 2 and 3, 1 and 4, are even entirely expelled from 

 the peristome, and in Atraj^us grandis^ Moera atroposj and Mi- 

 craster cor-aayuinum those of the pair 1 and 4. It is especially 

 the paired ambulacra of the trivium II. and IV., of which the 

 peristomial plates, longer than broad and nearly wedge- 

 shaped in the young, in old examples are broader than long, 

 ^nd so depressed that, while in small individuals of Brissojjsis 

 lyrifera 4'6 millims. in length the anterior margins of the 

 peristomial plates in the interradia 1 and 4 regularly cor- 

 respond to two plates in the ambulacra II. and IV., they 

 receive three plates in older individuals. Thus here also a 

 movement takes place in the ambulacra towards the peristome, 

 between the interradia. It is only a little less in degree in the 

 unpaired ambulacrum. In the bivium it is otherwise. Plere it 

 is the two oldest plates that are the most pressed ; the following- 

 ones, even in the older individuals, preserve their elongated 



