LOCALIZED STAGES TN DEVELOPMENT. 120 



Strongylocentkotus DitoisAciiiEXsis. As sliown by Loven ('••-!), the interaiiilnila- 

 cral i)lates of Strongi/hcentrofiis (Jrohtic/iicusls ( (). F. M.) , are marked at a very early stage 

 by a single large spine boss (PI. 24, fig. 112). An older specimen. Fig. llo, shows an 

 extensive resorption of the first interambiilacral plate, caused l)y the encroai'limeiit of the 

 peristome, and a second sj^ine boss on the third plate of the right-hand column. The 

 simplest condition is at the ventral border, and passing doi"sally during growth new features 

 are progressively added. This same method of adding structural features has been dem- 

 onstrated in many Palaeozoic Echini by Dr. T. A. Jaggar, Jr., and the author ( '-'0). 



In a series of StroiKjiilocciitrotiis (lrob<(rhieiisls from Massachusetts Bay a se([uence 

 of stages in the development of plates has been followed. In the youngest of these 

 specimens measuring 7 mm. in diameter (PI. 24, fig. 114), there are eleven plates in each 

 cohunn of iuterambulacral plates. The plates of the ventral pcn-tion are ornamented by 

 u primary and several secondary sitiue bosses. The last formed or eleventh plate is 

 simpler, has only a single spine boss, and thus resend^les all the plates of the very young 

 individual (PL 24, fig. 112) and the youngest or last added plates of the older individuals 

 (PI. 24, figs. 115, 116). An older specimen (PL 24, fig. 115) measuring 25 nun. in diam- 

 eter has sixteen plates in the interambulacral columns. The tojjmost or youngest plate of 

 the right-hand column is ver}^ young and is smooth, having no spine ))osses. This feature 

 is not shown in an^- of the other figures of this species, but is characteristic of the plates 

 of very young sea-urchins. The youngest or sixteenth j^late of the left-hand column has 

 one primary spine boss resembling the eleventh plates of Fig. 114 and all the plates of 

 Fig. 112. The eleventh plate from the base of Fig. 115, which may be compared with 

 the eleventh plate of Fig. 114, differs from that plate in having many secondary spine 

 bosses in addition to the primary boss. In the growth of the sea-urchin the ])eristonie 

 increases in size by the enlargement of tlie plates of the corona and also (in most sea- 

 urcliins) by more or less resorption of the ventral border of the corona. The ratio of 

 enlargement by the growth of plates and ventral resorption of the same is not at ])resent 

 known, and it is possil)le that one or more plates of Fig. 114 would have been resorbed 

 before the specimen attained the size of Fig. 115. The eleventh plate of Fig. 115 is not 

 therefore certainly the exact equivalent of the eleventh plate of Fig. 114. It may l)e the 

 e(|uivalent; but if not, it is a younger, not an older, plate than any represented in Fig. 

 114, so that for present purposes the comparison of these plates is applicable. 



In an adult specimen, Fig. 110, measuring 52 mm. in diameter, there are twenty-two 

 plates in the left interamljulacral cohunn. We find again that the youngest or twenty- 

 second and twenty-first plates have a single sjiine boss and resemble the sixteenth plate of 

 Fig. 115, the eleventh plate of Fig. 114, and ah the plates of Fig. 112. It is also seen that 

 the sixteenth plate, wliich has one spine boss in Fig. 115, has several in Fig. 110. The 



