EEPORT ON THE ECHINOIDEA. 9 



CoUyritidae and the Cassidulidas, and has shown that the separation of the apical system into 

 a bivium and a trivium, which at first sight appears so important, is not accompanied by 

 corresponding changes in other structural features of the test. On the contrary, these 

 groups agree in having comparatively simple ambulacra, without great differences in the 

 size of the plates of the interambulacra, and are thus more closely related still to the 

 Echinoneidse of the secondary period, of which they appear to be at the present day the 

 representatives ; and it is by the changes taking place in the odd posterior inter- 

 amljulacrum that this group approaches at last more closely the recent Spatangoids. 



Loven is disposed to consider the second actinal plate of the right posterior inter- 

 ambulacral plates as made up of the plates 2:2. I am more inclined to consider the 

 first and second plate as the breaking up of the first actinal plate into two parts, 

 as a similar splitting up of the actinal plates frequently occurs in the very elongate 

 plates of the actinal plastron of other Spatangoids. From the very fact that in such 

 closely allied genera as Hemiaster and Faorina the second plate is, according to Loven 

 himself, made up in the one case of 2:2, and in the other 2:3, I am the more inclined to 

 look upon this as a mechanical result due to the irregular termination of the median 

 interambulacral line of the posterior pair of interambulacra compared to the anterior 

 pair, and thus far nothing has been traced in the growth of the young Spatangoids to 

 sustain the view taken by Loven. It seems to me, on the conti'ary, that it is in the 

 modifications of the odd posterior interambulacral area that we must look for the passage 

 between the two groups of Spatangoids which Loven has followed. These can still be 

 traced, but very imperfectly, in the other lateral interambulacra which are not as deeply 

 afiected by the change of form as the odd posterior interambulacral area. I am rather 

 inclined to look upon this heteronomy of the right posterior interambulacrum, which 

 Lovdn has noticed, as the last trace of the structural affinity of the Clypeastroids 

 to the Spatangoids, the more so from the existence of such a genus as Palceostoma, in 

 which we find in all the paired ambulacra two single actinal plates in succession, the last 

 trace of the zigzag arrangement of the actinal plates of the paired interambulacral spaces. 

 If, on the other hand, with this explanation of the modifications of the paired lateral 

 interambulacra, we trace the changes which the odd interambulacrum undergoes, we can 

 trace these directly either to the more or less central ^losition of the actinostome or to 

 the elongation of the test which greatly modifies the composition of the plates of the 

 actinal plastron, and this, it seems to me, is a more natural explanation of the hete- 

 ronomy existing in the Spatangoids than the ingenious homologies of the successive 

 soldering of the plates 2 :2 or 2 :3 advanced by Lov^n. 



Older writers on the Echinoidea have already insisted on the similarity of the plates 

 composing the different ambulacral and interambulacral areas in the regular Echinids, 

 and their difference in the so-called irregular Echinids. This division, which at first sight 

 seems so fundamental, is most artificial ; and when we carefully analyse such groups as the 



(ZOOL. CHALL. EXP. PART IX. — 1881.) I 2 



