130 ROBERT TRACY JACKSON ON ECHINI. 



lividus (text-fig. 128). The suranal plate does not fill the periproct as in the earlier stage after 

 Loven, but it lies opposite genital 3, as in Salenia (Plate 4, fig. 1). A few secondary periproctal 

 plates have developed as seen also in the next two figures. No genital pores have appeared, 

 but instead of a single pore in the madreporite, as in text-fig. 131, there are several pores and 

 these increase in later stages (text-figs. 133, 134). At a later stage (text-fig. 133) ocular I has 

 entered the periproct, but all other oculars are exsert. This stage may fairly be compared with 

 progressive variants of S. lividus (text-figs. 129, 129a) that also have ocular I insert. In a still 

 later stage (text-fig. 134) ocular I is fully insert and V is narrowly insert. This specimen, 

 though only 7 mm. in diameter, has taken on the adult character of the bivium insert. In its 

 detail it can be aptly compared with the extreme progressive variant of S. lividus (text-fig. 130). 

 The specimen from which text-fig. 132 was drawn is 4 mm. in diameter, while that from which 

 text-fig. 134 was drawn is 7 mm. in diameter, not far from twice as large, yet the apical disc of 

 text-fig. 134 is only a little larger than that of text-fig. 132. This shows, what was earlier stated 

 (p. 87), that the apical disc proportionally does not keep pace with the size of the test during 

 growth but with development becomes relatively smaller. This relative change is more marked 

 in early than in later stages of growth in which the relative reduction in this species is very slight. 



In order to test the age at which characters are taken on and to see what might be the range 

 of variation at different ages, a large series of young and adult specimens was collected. The 

 youngest series of 2.5 to 5 mm. in diameter came from several localities on the Maine and Massa- 

 chusetts coasts, but were mostly collected on Chelsea Beach, Massachusetts, by Mr. J. Henry 

 Blake, who kindly gave them to me. Succeeding this age, a series of specimens was collected 

 at Dumpling Islands, North Haven, Maine. This locality is a favorable spot as specimens are 

 abundant and attain an exceptionally large size, so that the range of size and presumably age 

 is c[uite full. The specimens were collected at spring tides, when with exceptionally low water 

 they could be easily picked off the rocks. All were taken from within a radius of about two 

 hundred feet and all were from as uniform conditions as possible. The specimens were meas- 

 ured with a steel caliper rule; 11,500 specimens from this locality were thus measured and 

 observed, and are divided into two series : what I call a developing series under 30 mm. in 

 diameter, and a developed series over 30 mm. in diameter. This is based on the fact that when 

 over 30 mm., the typical adult character for that locality was established by the percentage 

 having the bivium insert. Younger series of specimens in a progressively increasing degree 

 have less of the full character developed and a greater percentage of individuals of immature 

 character as shown in the table, p. 142. 



Taking up the developing series, it is safe to assume that at a very early stage all specimens 

 would have all oculars exsert, as in text-fig. 131. Specimens (51) 2.5 to 4 mm. in diameter, 

 the youngest I found, have 29 % with oculars all exsert and 71 % with I only insert. Of these 

 latter, many specimens had the ocular only barely reaching the periproct, indicating that it 



