NOMENCLATURE. 19 



system, considered of such primary importance, are constantly likely to mis- 

 lead us, by placing young and old in different genera. The changes in the 

 abactinal system are as varied, yet, when we have once ascertained what is 

 the range of variation for a group, the characters it affords are of the utmost 

 value. In the irregular Echini, where the changes during growth are very 

 marked, we find genera and species based upon characters the value of 

 which is not the least understood. The shape, the position of the anus, the 

 structure of the ambulacra, have always been considered essential ; yet em- 

 bryology teaches us that nothing varies more during growth than the outline, 

 that the anus may be placed almost anywhere during the growth of the in- 

 dividual, and that the ambulacra may at one time be identical with those of 

 the regular Echini, and pass through all intermediate changes to the petaloid 

 state. We find, among Spatangoids, in the position of the apex, that of the 

 mouth, and in the presence of the fascioles, features of primary importance ; 

 with Clypeastroids the structure of the interior proves thus far our safest 

 guide ; while among the Echinolamps we come upon a group beset with dif- 

 ficulties, where everything seems variable, and the changes passed through 

 from young to adult would warrant placing the different stages of growth in 

 each of the principal subdivisions established among Echini. 



All our characters are variable ; the greater our range of comparison, the 

 less our standards become fixed or stable. How can we denote all this ? Are 

 we prepared to use a notation which will express these changes and be in- 

 telligible ? Can we do more than give a rough sketch of such a condition of 

 things ? Our notation must be the growth of our knowledge, and its meaning 

 and application must be simple ; in spite of the definite existence of what 

 we call species, genera, etc., when we apply these terms to limited regions 

 and series of the present clay, yet we find them totally inadecpaate to express 

 our wider interpretation when our standards of comparison are infinite in 

 time or space. 



It is found impossible in practice to determine when a species is sufficiently 

 described or not ; hence no attempt can be made to discriminate in favor of 

 this or that name on account of the character of the description. What 

 was ample to separate the few species of Clypeaster known at the time of 

 Lamarck becomes useless when describing species of the genus in 1870. 

 From insufficiently described species (or so considered) we pass imper- 

 ceptibly to mere catalogue names, known only from specimens deposited 

 in public museums, or by the distribution of casts, or of types ; finding 



