ECHIXID.^. 239 



arrange them in genera, which shall be at the same time, natural and sharply 

 defined. For few groups show more clearly than do the Echinidte, the pro- 

 gressive differentiation of species and while it is possible to trace out the probable 

 hnes of development, it is exceedingly hard to arrange the species in genera in 

 such a way as to indicate those lines. Taking any single character as a standard 

 we can indeed make lines of division sufficiently sharp to be easily seen, but such 

 a course works havoc with the natural relationships. Moreover some of the 

 most important characte: . show such a perfect series of steps from the simple 

 to the specialized condition, even within the limits of a single species, that they 

 are of little value in defining genera, or at least, must be used with great caution. 

 As an illustration of this point reference may be made to the tuberculation of 

 the ambulacra. Mortensen ("Ingolf" Ech., pt. 1, p. 93) first called attention 

 to the marked difference between having a primary tubercle on eaeh ambulacral 

 plate, and having one, only on each second, third, or fourth plate. Associated 

 as this is with a change in the form of the plates and the arrangement of the 

 pores, it is indeed a most important character. The use of it however as a 

 factor of primary importance in the classification of the family is nullified by the 

 fact that in Echinus acutus, specimens may be found in which every ambulacral 

 plate has a normal primary tubercle, while in others not only are many ambula- 

 cral plates smaller than their fellows and lacking a primary tubercle, but some 

 of the abactinal ones are actually made up of only two elements, a most unusual 

 condition in this family. Between these two extremes, all possible intermediate 

 stages are found. An attempt therefore to define genera with reference to the 

 tuberculation of the ambulacra, necessitates putting some specimens of acutus 

 in one genus and some in another, while some would be exceedingly hard to 

 place. A similar difficulty arises when stress is laid on the condition of the 

 gill-cuts. Thus in Lytechinus ^ variegatus, the adults have deep and well-marked 

 gill-cuts while in L. semituherculatus they are smaller and less distinct and in 

 L. verruculatus they are scarcely specialized at all. Moreover as Mortensen 

 has pointed out ("Ingolf" Ech., pt. 1, p. 115) in small specimens of variegatus, 

 the gill-cuts are no more noticeable than in many other species. There can be 

 no question however, that the absence of primary tubercles on many ambulacral 

 plates and the presence of sharply defined gill-cuts are evidences of specialization 

 and while their occurrence in Echinus and Lytechinus respectively is not of any 

 value for the definition of those genera, in Toxopneustes and Tripneustes, where 

 they have become fully differentiated and fixed, they are important generic 

 characters. 



1 For the use of this and other generic names, see below under the respective genera. 



