66 ROBERT TRACY JACKSON ON ECHINI. 



column of plates in each of the five areas. In the second row there are two plates, in the third 

 three, in the fourth four, in the fifth five, and so on. There is not always a new column added 

 in each new row or zone of growth in this species, but this is very nearly the case up to the 

 maximum number of columns added, which is eleven. Dorsally the adradial columns 1 and 2 

 drop out, so that columns 3 and 4 come against the ambulacrals in further dorsal growth. The 

 same series of stages of progressive development marked by adding columns, and regressive 

 senescent stages marked by the dropping out of columns of interambulacral plates are shown in 

 Hyattechinus rarispinus (Plate 23, figs. 1, 3) and H. pentagonus (Plate 25, figs. 1, 3). In this 

 last species in senescence, several columns are dropped, so that close to the apical region there 

 are ten columns instead of the fourteen characteristic of its full development near the mid-zone. 

 H . pentagonus is also especially interesting, as it attains fourteen columns of plates in the adult 

 in each area, the maximum specialization in this line of differential development of all known 

 Echini. Yet as resorption has not removed any plates, we can trace the development from the 

 simple ventral or young condition, represented by a single plate on the peristomal border, to 

 the complex adult and senescent old age without a single gap in the series of stages in develop- 

 ment. In other Palaeozoic genera a similar progressive individual development may be traced 

 in which the single ventral plate is retained in the adult, as in Pholidechinus (Plate 28, fig. 1), 

 Pholidocidaris (Plate 73, fig. 6), Lepidocentrus (Plate 20, fig. 7), and Lepidesthes (Plate 68, 



fig. 3). 



The above represent Palaeozoic genera in which there is no ventral resorption of the 

 corona, and therefore all the interambulacral plates, built during the life of the individual, 

 are retained and can be seen in a well preserved adult individual. Such successive stages in 

 the development of an interambulacrum can be fairly compared to studying the development 

 of a cephalopod or other mollusc, as has been done so successfully by Hyatt and others, by 

 tracing the characters from the umbo or original apex of the shell by a series of steps or stages 

 to the completed adult or old-age individual, all worked out in a single specimen. There is 

 the great difference, however, that in the mollusc the shell is external, in the sea-urchin internal, 

 so that in the latter the plates have grown since first formed and may have suffered changes, 

 but from their presence stages can be read. 



Resorption, however, comes in in Palaeozoic as well as in post-Palaeozoic types. Studying 

 genera representing the Palaeechinidae, we find that there are two plates at the ventral border 

 of the interambulacra in all cases where the ventral part of the corona is complete, as seen in 

 Palaeechinus (Plate 30, fig. 3), Maccoya (Plate 33, fig. 1), Lovenechinus (Plate 43, fig. 1), 

 Melonechinus (text-fig. 25; Plate 56, fig. 2; Plate 57, fig. 1), and others. This character is 

 explained by the fact that the single initial ventral plate, found so universally in young modern 

 Echini, and retained ventrally in numerous fossil and living genera, has been resorbed in the 

 advancement of the peristome, as in Cidaris and most modern regular Echini. Miss Klem 



