CLASSIFICATION OF PALEOZOIC ECHINI. 239 



Subclass Euechinoidea. — The class Echinoidea is divided into two sub- 

 €lasses — one, which is limited to the Paleozoic, except for the Triassic 

 genus Tiarechinus, and the other, which is known from the Permian 

 to the present time. On the accepted bases of classification this sec- 

 ond class, the Euechinoidea, in its simplest members, as represented 

 by Cidaris, is nearest to the primitive, as expressed in the embryo and 

 the genus Boihriocidaris, of any member of the group. The Euechi- 

 noidea are represented, as at present known in the Paleozoic, only by a 

 few species of the genus Cidaris ; therefore no further consideration is 

 given to this group. 



Subclass Palseechinoidea — Order II, Perischoechinoida. — In its normal 

 types this order presents a distinct, continuous series, varying in the line 

 of progressive addition of columns of interambulacral or ambulacral 

 and interambulacral plates, with a greater or less resorption of the 

 corona by encroachment of the peristome. 



Melonitidse. — Ambulacral areas, as stated, are considered the most im- 

 portant structural features ; therefore as the family Melonitidse have 

 the greatest advance in the line of progressive complication of this part, 

 they are considered the most highly evolved group of the Paleozoic 

 Echini. In considering this family careful attention is called to the dia- 

 gram, figure 1, page 191. 



Rhoechinus (see figure 1, page 191, and plate 7, figures 36, 37 and 40) 

 is the simplest genus of the family, having but 2 columns of ambulacral 

 plates in each area. The simplest species has 4 columns of interam- 

 bulacral plates, and the most highly specialized species has 8. This 

 highest species, R. gracilis (plate 7, figure 36), in area C has a ninth col- 

 umn near the dorsal area, but as it originated so late and in only one 

 area we should not call it a character of the species unless found in other 

 specimens. 



Palseechinus (see figure 1, page 191, and plate 7, figures 38, 39) is fur- 

 ther specialized than Rhoechinus, having the beginning of that condi- 

 tion of multiplication of ambulacral plates which is characteristic of the 

 family (see diagram, page 191). We cannot say that there are 2 columns 

 of plates in the ambulacrum, and neither should we like to call it 4; 

 therefore in the table the ambulacral columns are expressed as 2-f . The 

 simplest species has 5 columns of interambulacral plates, and the most 

 specialized species has from 6 to 7 in each area. 



Oligoporus (see figure 1, page 191, and plate 6, figures 25-34, and plate 9, 

 figures 50-52) is the third genus in the family and is quite well known 

 from a considerable amount of good material. The ambulacra ventrally 

 consist of 2 columns of plates, which pass into 4 columns in the adult ^ 



* In Oligoporus dance (plate 6, figure 30) there are a few additional plates in the middle of each 

 half ambulacrum. 



