414 PROF. F. J. BELL ON THE ECHINOMETRID^. [Mar. 15, 



growth of the pore-plates, the distinction between them altogether 

 breaks down ; whether that distinction be physiological or morpho- 

 logical is, then, an unnecessary question. 



Coming next to the absolute distinctness of the groups as indi- 

 cated by the number of the pairs of pores, we are met, first of all, 

 by the considerations which surround the vexed questiou of the 

 value of any delimitation by the absolute use of definite numbers. 

 On the one hand, it is quite certain that a classification of the 

 Asteroidea which depends on the number of the rays would exhibit 

 a very incomplete account of the systematic relations of the members 

 of the class ; but, on the other hand, it is just as true that no better 

 name was ever applied to the winged Insecta than that of Hexapoda, 

 or to the higher Vertebrata than that of pentadactyle ; and it is just 

 as clear that the division of modern Ungulates into two groups, one 

 perissodactyle and the other artiodactyle, could only have beeu 

 suggested by a naturalist capable of seeing a great general truth 

 through a not always constant similarity in detail. 



We now have to weigh these two opposing arguments in applying 

 to the Echinidse (of earlier writers and of Love'u) the mode of 

 classification suggested and worked out by Desor', by which we get 

 the two groups of the Oligopori and Polypori. The test to be ap- 

 plied shall be twofold. First, let us see how it works in the hands 

 of so skilful a naturalist as Prof. Alex. Agassiz. His division of the 

 Echiuometradae is defined (as we already know) as, inter alia, 

 always having more than three pairs of pores to each arc. But, as 

 a matter of fact, he includes under the Echinometradse the two 

 genera Parasalenia and Echi7\ostrep.(iu.s. Of the former he says 

 " this genus seems to be an Oligopore among the Echinometradse, 

 having but three pairs of pores in each arc." In speaking of 

 Eclnnostrephus the generic definition includes no reference to the 

 number of pairs of pores in an arc ; but in speaking of E. niolare, 

 the only species of the genus, he says " there are from three to four 

 pairs of pores iu each arc, the majority having but three pairs." 



The other consideration arises from a study of the facts as ex- 

 hibited in the tests of various species. If in any of these some of 

 the arcs can be shown to possess only three pairs of pores, it seems 

 to me that such a fact alone would disqualify numerical relations 

 from forming the criteria of generic, or even higher, delimita- 

 tions. 



Turning again to the guide we have already followed, we find this 

 sentence : — " Le quatrieme arc, ici muni de quatre pores, n'en a que 

 trois chez quelques individus du Toxopneustes drobachiensis, c'est-a- 

 dire que la plaque composee 3 ne possede qu'une seule plaque 

 primaire mediane. II y a done quelque variabilite."^ 



So, again, Dr. Liitken finds in the rare Echinometra oblonga that, 

 towards either pole of the corona, there are but two or three pairs 

 of pores in each arc^. 



' Synopsis ties Ecli. fuesilcs. '' T. c. p. 25. 



' Cf. tig. 10 ol the fii-st plate iu his 'Bidrag til Kmidskub om Echiuoderme,' 

 1864. 



