Prof. S. Lov^n on the Structure of the Echinoidea. 435 



the same group, are narrowed or pointed backward ; and their 

 outer margin forms, with the outer margin of the first abdominal 

 pair on each side, tlie angulus ejnsternalis^ into which the pro- 

 duced plates of the bivium project. This angle is very deep 

 in EupataguSj Breynia, PlagionotuSj Maretia, EcMnocardium, 

 Xanthobrissics, and Palceotroj^usj and shallower in Lovenia, 

 BrissuSj Meoma, Brissopsis^ and Sjjatangus. It is very small 

 in the Micraster of the Chalk, in which it makes its earliest 

 appearance, and which has the epistemal plates still of nearly 

 the same form as the other abdominal plates ; this is also the 

 condition in the Prymnadeti, in which all the posterior part of 

 the unpaired interradium (e. g. in Hemiaster)^ by its uniformity, 

 has a certain resemblance to the same part in the Cassidulidse. 

 This group of Spatangidte also differs from the Prymnodesmii 

 in that the plates of the unpaired interradium belonging to the 

 left side are notably longer than those of the right side. Even 

 the left sternal plate projects beyond the right one in most of 

 them ; if not, the episternum and all following segments are 

 elongated. InPalceostoma this difference goes very far, inasmuch 

 as the left plate of the episternum lies with nearly the whole of 

 its length behind the right one, and all the following nearly in 

 the same way. A consequence of this is a projection by which 

 the periproctium, which here also is bounded, at least in front, 

 by the same plates during the whole life of the animal, is not 

 always symmetrically enchased — that is to say, that on one side 

 a smaller number of plates attain its margin ; and then it is 

 always the left side that is furnished with a plate more than the 

 right side. In general we may observe that in the Spatangidee 

 the left side is preponderant. 



If we compare a very young 5r^s50J?5^s lgrtfera,4'Q millims.in 

 length, with a full-grown one, we find that the paired interradia 

 in the former have already acquired their permanent form, only 

 that the oldest plates (of the peristome) are comparatively 

 broader, and plates 2 a little longer than in the full-grown spe- 

 cimen — and that in the frontals the small individual has eleven 

 or ten plates, and the large one thirteen, two small plates having 

 been added at the ends. In the laterals ten plates may be 

 counted in both. The unpaired inten-adium has undergone far 

 more change. The labrum and sternum are alike in both ; the 

 episternum in the small individual very short, its posterior 

 margin very narrow ; and the prseanal plate, that which does 

 not enter into the periproctium, and which changes most of all 

 the plates of the test, is still three times as long as broad, while 

 in the adult its length and breadth are nearly equal. In the 

 latter there are 5-8 anal plates, in the young 5-9 ; the whole 

 number of plates in the young is fourteen, in the adult fifteen. 



