1881.] PROP. F. J. BELL ON THE ECHINOMETRIDvE. 411 



As subgenera of Strongylocentrotus, Prof. Alex. Agassiz includes 

 Sphcerechinus and Pseudoboletia, the former of which Dr. Gray 

 would appear to have included with Echinus in his fifth family, 

 while the latter is a genus of which no species was then known. 

 Echinostrephus had not, in 185.5, been distinguished from Echinus 

 or Psammechinus ; while Siomopneusies, under the title of Helioci- 

 daris, was also regarded by Gray as closely allied to Echinus. Of the 

 nine genera, or subgenera, found iu the family of the Echinometradse 

 of Agassiz, viz. (I) Colobocentrotus, (2) Heterocentrotvs, (3) 

 Echinometra, (4) Parasalenia, (5) Stomopneustes, {Q) Strongylo- 

 centrotus, (7) Sphcerechinus, (8) Pseudoholetia, (9) Echinostrephus, 

 the first three and the sixth alone fall into Gray's family, the fourth 

 and the eighth were unknown to science, while a different view was 

 taken as to the affinities of Stomopneustes, Sphcerechinus, and Echi- 

 nostrephus. They were regarded, in fine, as being more closely allied 

 to Echinus, because they have the " ambulacral area half as wide as 

 the interambulacral area, with two (or three) close series of double 

 pores, placed in threes ; buccal membrane naked ; body circular." 



We may dismiss the first character, without even examination ; for, 

 while it is obviously artificial, it is the same for Gray's two groups 

 of Echinidae and Echinometradse. As to the second difference, the 

 arraugement of the pores, there can be little doubt that, judging 

 by it only, Stomopneustes has a much closer affinity to the Echino- 

 metridse than to the Echinidae. And we now come to what is really 

 the kernel of the whole matter. How far is Desor's division into 

 Oligopori and Polypori natural 1 and how far is it artificial 1 



If we examine one of the least modified of the Echinidae, e. g. 

 Cidaris iribuloides, we find that the pores of the ambulacral zones 

 are arranged regularly and equally in pairs, are, in effect, set one 

 behind another in a straight line, and belong each to a single simple 

 plate. If we take a more modified form, such as a species of the 

 restricted genus Echinus, we find the pairs of pores have, for the 

 greater part of the test, come to be set in arcs of three ; and on close 

 examination it is seen that the plates connected with these pairs of 

 pores are not all of the same size, and that the primary plates fuse to 

 form a secondary plate'. 



This is the typical arrangement among the Oligopori ; but it by 

 no means holds for all the plates ; those nearest the apical area 

 have, more or fewer, the pairs of pores in just as straight lines as 

 Cidaris tribuloides. 



Taking, as an example of the Polypori, Echinometra subangu- 

 laris, we have some six pairs of pores arranged in a much more 

 elaborate arc, and the changes that come to be effected are so great 

 that what form really the distal pair of pores of one arc seem to be 

 the proximal pair of the succeeding arc. 



' It seems to me that all the advantage lies in continuing to use the nomen- 

 clature of Johannes Miiller, and to speak of the first or simple plates as frimary, 

 and the fused plates as secondary ; for reasons wliich, no doubt, are excellent, 

 Prof. Alex. Agassiz has (pp. cit. pp. 642, 643) elected to reverse this nomencla- 

 ture, and to speak of the compound plate as the priuiarv one. 



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