92 Dr. Tij. Mortensen on some Echinothurids 



tliat be a somewhat unjust reproacli ? The diagnoses of genera 

 must, of course, be made from a comparative study of the 

 species known, and then it has to be left to the authors 

 describing new species to emend the diagnoses if the new 

 species prove the old diagnoses to be too narrow or too wide. 

 Such lias always been and will remain the case, as it 

 cannot be demanded that one should guess beforehand the 

 characters of species as yet unknown. It may well be said 

 to be a drawback to our classifications, but it is common to 

 all of them, and not peculiar to my classification of Echinids. 

 "When, further, de Meijere says {loc. cit.), " Das System 

 iiihrt also zu grosser Zersplittering, was wohl den Vorzug hat, 

 class nur das sehr ahnliche zusammen bleibt, aber auch den 

 Naclilheil, dass die docli nahe Vcrwandfschaft mit der abge- 

 trennten Form aus dem Genus-Namen nicht mehr zu erkeinien 

 ist,^' I might remark that it is allowed and desirable to 

 make the new generic names such tiiat they indicate to which 

 old genus the new one is allied — for instance, let the names of 

 Cidarids end in -cidaris^ those of Diadematids in -diadema, &c. 

 The principal object, however, is to get the genera, and above 

 all the species, distinct and clear, 'i he species are the units 

 with which we must work^ and when these units are com- 

 })Osed of different things — as was the case with several of 

 the Echinid species — much labour will be lost {e. g., on the 

 geographical distribution) . The arrangement of the species in 

 genera and the genera in families &c. is of secondary import- 

 ance, and differences of opinion on this subject are of far less 

 wide-reaching consequences. 1 may cite, in conclusion, the 

 wordsof L. Agassiz*: — "Loin d'etie nuisible aux vrais progr^s 

 de la science, cette multiplication des genres, lorsqu'ils sont 

 dtablis sur des caract^res prdcis, ne saurait avoir d'autre effet 

 que de rapprocher de ])lus au plus les es| feces que leurs 

 caraci feres naturels lient le plus etroitement. C"est-1^ le 

 grand avantage des jietits genres, et cet avantage est surtout 

 sensible dans les families, dont toutes les esj feces se ressemblent 

 par leur aspect exteiieur et par Pensenible de leurs caracteres.^'' 



EXPLANATION OF THE PLATES. 



[The microscope used was a Zeiss instrument.] 

 Plate II. 



Fiff. 1. ArcBosoma Owsteni, from the actinal side. 

 Fig. 2. Same, from the abactinal side. 



Introduction to Valentin's ' Anatomie dii gf^nre Ec/unus,' p. x. 



