230 R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



plate as seen in young stages of other genera * There is at work a con- 

 tinual resorption and renewal, a taking on of one form and losing it for 

 another, etcetera. This at first sight seems to work havoc with any pos- 

 sibility of following stages in growth from an adult corona, but we think 

 not for the Palreechinoidea in most cases, at least. Resorption takes place 

 principally at the ventral border of the corona, being a consequence of the 

 encroachment of the peristome. This is markedly seen in Strongylo- 

 centrotus (plate 3, figures 8, 9), Cidarls and ArcJixocidaris (plate 8, figures 

 43, 47 and 48. and plate 9, figure 55). In other cases little or no ventral 

 resorption takes place, as in most of the Exocyclica, Pholidocidaris 

 (plate 9, figure 54) and Lej)idechi)ius (plate 7, figure 42). Resorption in 

 other parts of the corona as understood would for the most part be the 

 resorbing of one set of surface spine bosses (and concurrently the spines 

 themselves, probably) and replacing them by another set of perhaps dif- 

 ferent form or number, as in Cidarls (discussion, page 215). It is possible 

 that the outline of plates may change in this process of resorption and 

 growth. For example, in Melonites (plate 3, figure 14) the initial plate of 

 column 8 when young may have had the normal pentagonal form, as in 

 figure 15, and later have taken on the peculiar hexagonal form shown in 

 figure 14. This is a kind of change which we cannot prove one way 

 or the other from the study of the adult. The plates themselves only 

 disappear by resorption very rarely. Professor Loven has shown in his 

 ^' Etudes " (plate 51) that in the adult of Ararhno ides placenta L. the single 

 initial plate of the interambulacra is retained in only one area, whereas 

 in the young this plate exists in all 5 areas. At the same time the initial 

 row of primitive ambulacral plates are retained throughout life. This is 

 a case in which 4 initial interambulacral plates have apparently disap- 

 peared by intercoronal resorption. No otlier case of removal of plates 

 of the corona by resorption has been met with excepting that caused by 

 the encroachment of the peristome. On account of the very definite 

 form of coronal plates seen throughout species, genera and families of 

 Paleozoic P]chini, I think it may be properly assumed that while plates 

 grow by enlargement, tlie relative form of plates in this group is constant 

 throughout the life of the individual. 



Professor Loven (25) adopted a system of notation of interambulacral 

 plates of the corona in which he numbers the several plates as added 

 according to the row in which they occur. The system of notation 

 adopted in this and the preceding paper differs from his in that the 

 column rather than the row is the basis of numbering. In the accom- 



*The progressive increase in size of this first plate, without anj' considerable change in fornn I 

 have traced in a quite complete series of developing Echinarachnius parma, varying from a few 

 millimeters in diameter to the completed adult. The same fact may be traced in a number of Pro- 

 fessor Loven's figures of developing Echini (see his "Etudes"). 



