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



with a single plate, 2 and 4 with two plates ; and if we reduce 

 the peristome of Echinoneus to a circle, this arrangement also is 

 symmetrical to the diameter which passes through ambulacrum 

 I. and interradium 3. All other irregular Echinoidea have in 

 each interradium only a single plate in the peristome. The 

 arrangement of the interradia is symmetrical in relation to the 

 animal's antero-posterior axis, which is also its longitudinal 

 axis, and passes through the unpaired ambulacrum III. and 

 the unpaired intenradium 5 — with a constant deviation in the 

 Spatangif^ze, without deviation in the Clypeastridfe and Cassi- 

 dulidas ; whilst in these two families all the four paired 

 interradia resemble each other in a high degree, the two frontal 

 above all, and the two lateral are reciprocally perfectly sym- 

 metrical, and the unpaired interradium differs little from 

 them, the plates being analogous in proportion and form, but 

 cut out as if eroded for the periproctium, which is surrounded 

 by the same plates during the whole life of the animal. This 

 is the case in a high degree in Laganum and EchinocyamuSy 

 which have one of the youngest plates which pushes into the 

 vertical plate large and pointed. So also in Glypeaster^ 8to- 

 lonoclypus^ Encope^ Mellita^ Echinarachnius, and ArachnoideSj 

 in which the younger plates are gradually smaller and smaller, 

 and those which lie close to the vertical plates small and of 

 equal size, diverging and receiving the genital pores between 

 them. 



Much of all this is quite diiferent in the Spatangidae. The 

 frontal interradia are symmetrical in all living genera ; and 

 between them and the lateral ones there is a considerable 

 agreement. If the plates 2 in the frontals are very large, and 

 the following ones very short, as in Breynia^ Jjovenia^ Eupa- 

 tagus^ PlagionotuSj Maretia^ Spatangus, and Echinocardmm, 

 the same conditions occur in the lateral interradia ; if the plates 

 of the frontals approach a nearly equilateral pentagonal or 

 hexagonal form, the same prevails in the laterals. But among 

 themselves the lateral interradia are never alike, always un- 

 symmetrical on both sides of the longitudinal line ; and it is 

 always the right lateral interradium, 1, that deviates. Those 

 Spatangidffi, which seem to be most numerous among existing 

 forms, but were very few during the earliest periods of the 

 family, the Prymnodesmn^ or those which have an infraanal 

 fasciola and the most regular ambulacra, are also those in which 

 this asymmetry is most strictly maintained. All their genera 

 have in the right lateral interradium, in its hinder row, 1 «, 

 one plate less than in the same row of the left one, 4 h ; the 

 first three plates of the right peristomial plate 1 and two 

 following, represent the first four of the left lateral interradium, 



