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



it : — in some to a small extent, as mMeoma ventricosa- in others, 

 e. g. BrissbjJsiSj to a greater extent ; and in some, e.g. Schizaster 

 fragilis (Diib. & Kor.), so completely that the genital pore is 

 absent, and with it the right anterior interradial genital gland. 

 The next in order to disappear are the genital pore and gland 

 of the left anterior genital plate; and when, as in Abatus 

 PhilijJinij n., and Palceostoma mirabile^ Grray, only two genital 

 pores remain, these are situated in the lateral genital j)lates. An 

 aiTangement by which the madreporic plate extended backward 

 separates from each other the eye-plates of the bivium, occurs 

 among the Sixitangidce of the Eocene period, and not only in 

 most of those which also belong to recent times, but also in 

 some (e. g. Prenaster^ Ifacrojjneustes) which had abeady made 

 their appearance in the younger deposits of the Cretaceous 

 formation. On the other hand the genera which essentially 

 belong to the latter formation and attained in it their highest 

 development, present throughout a different disposition of the 

 genital and madreporic plates, at the same time that the latter 

 does not reach the posterior interradium, but is separated from 

 it by the eye-plates of the bivium, which meet and touch each 

 other, as do also, in most, the lateral genital plates. Among 

 the known living Si^atangidai only one has this character of 

 antiquity, namely liemiaster exj^ergitus^ n., Avhicli was dis- 

 covered on the voyage of the Swedish corvette ' Josephine,' in 

 the year 1869, by Smitt and Ljungman, near Josephina's bank, 

 in 38° 7' N. lat. and 9° 18' W. long., at a depth of 550 fathoms 

 on a clay bottom. The genus, which until then was regarded 

 as having become extinct during the Miocene Tertiary period, 

 and which attained its highest development during the Cre- 

 taceous period, is recognized by its rounded oval outline, 

 which, with a length of 14 millims., has a breadth of 13 millims., 

 by its posteriorly considerable height (10 millims.), by the posi- 

 tion of the periproctium high up on the posterior surface, the 

 single peripetalous broad fasciola, which forms an oval ring, 

 the short, broad petala of the bivium half as long as the an- 

 terior ones, and, for still further distinction from Abatus^ by 

 the madreporic plate with which the anterior right genital 

 plate is united, but which posteriorly does not reach the unpaired 

 interradium, but is shut off from it by the two eye-plates of 

 the bivium and the lateral genital plates. The individual is 

 young, so that the four genital pores do not yet perforate the 

 genital plates, and the madreporic plate has only a few pores; 

 but the peristome is reniform, and the lobes prominent. The 

 ambulacra are remarkably narrow where they pass under the 

 fasciola. The test is extremely thin and brittle. 

 In the regular Echinoidea the anus opens in the circle formed 



