200 R. T. JACKSON — STUDIES OF PALiEECHINOIDEA. 



Meloiiitidse, because the species as a whole and some individually are 

 farthest removed from the primitive in the direct line of variation of the 

 group. It may also be considered the most extreme known form of 

 Palaeechini in the line of plate multiplication, having in both ambulacra 

 and interambulacra more columns of plates than any other genus.* 



The systematic intercalation of new columns during growth necessarily 

 renders the number of columns a somewhat unsafe specific character 

 where the description is based on imperfect materials, for a more com- 

 plete specimen might show that higher up more than the supposed num- 

 ber of columns existed ; also, as new columns were added during the 

 development of the individual and mark stages in growth, the number of 

 columns is a criterion not of specific differentiation only, but of age as 

 well t (see page 188). 



Consideration of Rhoechinus and Palmechinus. 

 notes on earlier studies. 



The genus Rhoechinus was founded by W. Keeping (22) for a single 

 species, R. irregularis, W. Keeping. Dr Duncan (8) in a critical study of 

 ambulacral areas of PaLxechinus divided that genus. He transferred to 

 the genus Rhoechinus those species which have but one vertical row of 

 pores in each half ambulacrum, as R. {Palxechlnus) gracilis (plate 7, 

 figure 37) and R. (Paheecltinus) elegans (plate 7, figure 40). In the older 

 genus, PaLxechinus pars, M'Coy, he retained those species which have 

 a double vertical row of pairs of pores in each half ambulacrum, as in 

 Palxechinus gigas (plate 7, figure 39). J 



This division of the genus PaLxechimts is most satisfactory and is in 

 accordance with the relations expressed in the progressive development 

 of ambulacral plates in the Melonitida? (figure 1, page 191). ^\llile Dr 

 Duncan considers this diff'erence of the two genera, he does not state the 

 fact that on account of this diff*erence Rhoechinus is more simple in its 

 structure and Palxechinus is more sjiecialized, having made first steps in 

 the line of increase of ambulacral columns of plates. 



I cannot agree with Dr Duncan (9) in putting Rhoechinus and Palx- 

 echinus in the famil}'- Archa)ocidaridte. The fact of two columns of ambu- 

 lacral plates in each area seems insufficient evidence for such grouping 



*In one species, Lepidesthes colleti White (41), this number of ambulacral plates is exceeded, for 

 this species has eighteen, and perhaps twenty, columns of ambulacral plates according to White. 



fThis is opposed to Messrs Miller and Gurley's view. See page 138. 



X I would state that I had independently arrived at tiie same conclusion in regard to the ambu- 

 lacral plates in M'Coy"s old genus Palceechinus as Dr Duncan. In fact, the manuscript was written 

 for the succeeding sections, describing Rhoechinus and Paloeechinus, and all accompanying figures 

 were drawn before seeing his paper. Practically the only changes introduced are the substitution 

 of Rhoechinus for Palceechinus where thtit name applies. 



