178 SYNOPTIC COLLECTION. 



completely before reaching the anal area. Only a few 

 specimens with this specialized character have been 

 observed. In this species the new rows are introduced 

 early in life, showing that the law of acceleration in 

 development is in operation. The Melonite form is also 

 much more pronounced than in Melonites multiporus. 



The anal disc of Melonites is surrounded by the ring of 

 alternating genital and ocular plates. The five genital 

 plates can seldom be seen in specimens although well 

 preserved in No. 320. These plates are pierced by holes, 

 while the ocular plates, according to Jackson, are without 

 perforations. l 



It is seldom that the history of a group can be made 

 out by the study of a portion of the adult of a single 

 genus, but we have already seen that such is the case 

 with Melonites. The primitive condition of Bothrioci- 

 daris, the successive progressive stages of Rhoechinus, 

 Palaeechinus, and Oligoporus are all represented in the 

 ventral border and in one ambulacral and one interambu- 

 lacral area of Melonites. Nor is this all ; the greater 

 specialization by the process of reduction is illustrated by 

 a few specimens of this genus. 



We have seen that Goniocidaris and Cidaris are 

 among the most primitive of living Echinoids. Alexander 



1 Meek and Worthen (Geological Survey of Illinois, II, 1866, p. 

 228) state that the ocular plates of Melonites multiporus M. & W., 

 are without any traces of pores, and the figures are drawn without 

 them. In a footnote, however, they add, since the above was 

 written, we have examined "another fine specimen showing the 

 disc. In this there are four ovarian pores in three plates, and 

 three in each of the other two, while in two of the ocular pieces 

 there is apparently a single pore near one side." 



Roemer figures the oculars with two pores (see Arch. f. Naturg., 

 I, 1855, pi. xii, fig. 4). In the text he says, p. 322, ''The number and 

 position of the pores in these [ocular plates] cannot be recognized 

 with complete certainty, yet, there are apparently two of them in 

 each plate and at the same height as those in the larger plates" [geni- 

 tal plates]. 



