ECHINOIDEA. II. 



the vertex being at the end of the posterior petals. The sides of the test are almost vertical, the 

 actinal side almost flat. The periproct is situated near the abactinal side, in a slight furrow. The 

 ambulacra are very little deepened. - The young specimens are somewhat more egg-shaped, but the 

 posterior end is high as in the larger specimens, the outline in profil being mainly the same; only 

 in the specimen of 3 mm length the posterior end is yet rather sloping, the anal opening being very 

 near the apical system. 



To give a detailed description of the structure of the test would be superfluous after the 

 elaborate analysis and figures given by Loven, only a few additional remarks can be given on 

 account of the larger material at disposal. In the younger specimens the peristome and mouth is 

 as yet quite embryonal, the labrum not prominent at all; in the specimen of 3 mm the peristome is 



Fig. 16. Peristome and adjoining part of the 



test of a young Hemiaster expergitus, 3' nm in 



length. 2 5/i. 



Fig. 17. Apical system of a young Hemiaster 

 expergit-us, 3""" in length. s/i. The outline 

 of the smaller ambulacral plates and of some 

 of the inner interambulacral plates not quite 

 ' sure. 



quite pentagonal; the peristomial membrane is full of small somewhat concentrically arranged plates 

 (Fig. 16). In the larger specimens the labrum becomes by and by rather prominent, a little pointed, 

 with the edge a little thickened and reverted. Its posterior edge reaches, in the smaller specimens, 

 only to the middle of the adjoining ambulacral plates I. a. i and V. b. i (Comp. Loven. PL .46); in 

 somewhat larger specimens it reaches to the end of the first ambulacral plates, or a little farther on 

 the right side, as in Loven's Figure 114 (and his PI. V. 47), and in the grown specimens it reaches 

 to the middle or even to the end of the second adjoining amhulacral plate on each side (PI. II. Fig. 4); 

 generally the ambulacral plates of V. b. are a little shorter than those of I. a, so that the right side 

 of the labrum appears to reach a little farther than the left, but it is really symmetric. In the larger 

 specimens the inner ambulacral plates are comparatively much smaller than in the young specimens, 

 and their outline likewise is different. But though it thus looks rather different in the young and 

 grown specimens, no character for eventually distinguishing two species is to be found herein; it is 

 a difference due only to age, all transitional stages being found in the corresponding intermediate sizes. 



