Prof. S. Lovdn on the Structure of the Echinoidea. 433 



namely perlstomial plate 1 and the three following 2, 3, 4. 

 One of the three in interradium 1 a must therefore be regarded 

 as composed of two plates ; and it is clear that it is the second, 

 which consequently should be indicated by 2 + 3 ; but both 

 plate 1 and plate 4, and sometimes plate 5, assist in filling up 

 the deficiency which occurs when the right 2 + 3 is not so large 

 as the left 2 and 3 taken together. In most cases the former 

 is as large as the two latter, as in Brissoj)sis, Maretta, Lovenia, 

 EupataguSj and Echinocardium ; and in these, moreover, plate 

 4 is of equal size on both sides ; in Breynia^ Plagionotus, 8pa- 

 tanguSy Xanthobrissus, Micraster^ Paloiotropus, and Meoma 

 (which last has only a half infraanal fasciola) the plate 2 + 3 of 

 the right side is somewhat less than the left 2 and 3 taken 

 together ; and then it is the right plate 4 in Micraster^ Echino- 

 cardium, and Pakeotropus, and with this also plate 1, and, in 

 Meoma, plate 5, that is larger than the corresponding plate on 

 the left side and fills up the deficiency. In Bi-issus, on both 

 sides, plate 2 in Ih and 4 a, is so large and so nearly equi- 

 laterally pentagonal that it presses away plate 2 in 1 a and 4 b 

 from all contact with plate 1 ; and, moreover, plate 2 in 1 & is 

 much larger than in 4 a, so that it makes up no small part of 

 the measure that must be filled up in order to correspond with 

 2, 3, 4 in 4 5 ; here, also, 2 + 3 in 1 a is considerably less than 

 2 and 3 in 4 & taken together, and 4 in 1 a larger than 4 

 in Ab. 



The second group of Spatangidge, which is now, so far as we 

 know, less rich in generic forms, the Prymnadeti, those which 

 are destitute of an infraanal fasciola, is far less regular as 

 regards what is here in question. Five genera resemble many 

 of the Prymnodesmii in this respect — that they have plate 2 + 3 

 in interradium 1 a of the right side equal in size to the left 

 2 and 3 taken together, and, moreover, plate 4 of equal size on 

 both sides, as in Agassizia and ScMzaster, or, as in Mcera, 

 Abafus, and Hemiaster, Y>^ate 2 + 3 in 1 a smaller than 2 and 3 

 in 4 b, and therefore plates 1 and 4 of the right side, and in 

 Hemiaster also 5, somewhat larger than on the left. But three 

 other genera are very divergent. Desoria has plate 2 of inter- 

 radium 1 a amalgamated, not with plate 3 in the same row, 

 but with plate 2 in 1 i ; and the same occurs mAtrapus ] and 

 in the former plate 2 in 4Z» is besides separated from plate 1. 

 Palceostoma, however, is the most divergent. Whilst all other 

 Spatangidse have, in each interradium, immediately after the 

 peristomial plate 1, a pair of plates 2, this genus has plate 2 

 simple in both the frontal and lateral interradia ; and, moreover, 

 in interradium 1 it is not in 1 « that plates 2 and 3 are amal- 

 gamated, but in the row 1 b. 



