18 ROBERT TRACY JACKSON ON ECHINI. 



A similar beveling is seen in the petaloid area in some clypeastroids, Clypeaster, surely a 

 parallelism only. Accessory pores in genital plates in modern Echini exist as an individual 

 variation occasionally (Arbacia, text-figs. 197, 198, p. 171; Echinus and others). This appar- 

 ently' represents a parallelism rather than a genetic connection with the several pores charac- 

 teristic of these plates in most Palaeozoic genera (Plate 56, fig. 6). Increase in the number of 

 columns of interambulacral plates is taken on independently in many genera. The absence 

 of resorption of the base of the corona, which permits of the retention of the single pri- 

 mordial interambulacral plate in each area, appears as a parallelism in many independent 

 groups, as represented by Hyattechinus (Plate 26), Perischodomus (Plate 64, fig. 2), Pholi- 

 docidaris (Plate 73, fig. 6), Phormosoma, clypeastroids (text-figs. 43, 52, p. 80), and spatan- 

 goids (Plate 3, fig. 15). 



Resorption is an important process to bear in mind; it occurs continuously in the growth 

 of each individual plate, where there is a constant resorption within the plate concurrently 

 with increase in external dimensions. Resorption of the base of the corona by the encroach- 

 ment of the actinostome is a very important factor, as shown by Loven (1892). In Palaeozoic 

 types, as I showed in my earlier paper (1893), there is either no resorption (Hyattechinus, 

 Plate 26), resorption of apparently one plate only (Palaeechinus, Plate 30, fig. 3), or resorption 

 of several rows of plates (Archaeocidaris, Plate 9, figs. 6-8). Resorption may cut holes 

 directly through the test, as shown by Mr. Agassiz (1872) in the development of the lunules in 

 the recent Mellita sexiesperforatus (Leske). See text-figs. 22-31, p. 70. 



Variation is an extremely important subject and is graphically shown in sea-urchins. Here 

 we find the striking character of radial variation well developed, as seen especially in the corona 

 and ocular and genital plates. One radius may be more or less accelerated, or may reach a 

 greater or lesser degree of differential development than other radii in the same individual. This 

 is especially marked in the variation of rate of development and number of columns of interambu- 

 lacral plates attained in Palaeozoic Echini, and in the number of ocular plates reaching the peri- 

 proct in Recent Echini. The range of radial variation in one individual may equal the range 

 of variation of different individuals within the limits of the species. In IVIelonechinus typicallj' 

 there are four plates at the ventral border of the ambulacrum, but in one specimen of Melon- 

 echinus muliiporus (Plate 57, fig. 3) it is seen that in area B there arc two plates only. This 

 is the normal character of the ventral border of Oligoporus (Plate 50, fig. 8) and Lovenechinus 

 (Plate 42, fig. 1), the next lower genera of the family. 



In order to appreciate variation it is of fundamental importance to be familiar with the 

 characters of the associated species and genera of a case in hand, and also the developmental 

 characters of the same. Variation may be fairly classified under five more or less distinct 

 heads : 



1. Arrested variation, in which the variant retains characters seen in its own young and 



