64 ROBERT TRACY JACKSON ON ECHINI. 



Hyatlechinus beecheri (Plate 26) aiid Perischodomus biserialis (Plate 64, fig. 2). Complications 

 may come in, however, especially resorption of the base of the corona by encroachment of the 

 peristome cutting off part of the ventral plates, and also rarely resorption within the corona, 

 as exceptionally in Arachnoides, or differential growth of associated plates, which may separate 

 plates originally in contact (Echinarachnius). 



There may be few plates in a given interambulacrum, as in Bothriocidaris (Plate 1, fig. 1) 

 or Cidaris, or a very large number, as in Hyattechinus beecheri (Plate 26), and many other 

 species. There is every evidence that these plates all originated as separate plates and never 

 by division of previously formed plates. Mr. Agassiz (1881, p. 95; 1904, p. 104) saj's that 

 in Phormosoma, interambulacral plates are formed by the splitting of earlier continuous plates. 

 Mortensen (1904, p. 25) suggests that these planes of division may be due to fracture in the 

 handling of the specimens which have excessively thin plates, and my limited observations 

 of Echinothuriidae accord with this view. If these are natural suture lines in Phormosoma, 

 the numerous plates thus formed cannot bo homologized with the numerous columns of plates 

 in Palaeozoic types to which Mr. Agassiz compared them, for the latter originate independently, 

 not by the splitting of previously continuous plates. While fusion into compound plates is 

 seen in the ambulacrum of the Centrechinoida, such fusion is unknown in the interambulacrum 

 in all Echini. Loven (1874, p. 51) assumed, it is true, fusion of certain plates around the base 

 of the corona in spatangoids. No sutures exist, and it seems that single plates at this area 

 in these and other types are not cases of fusion, but of failure to develop more than one plate 

 in the row to which they belong. 



In the important ancient type Bothriocidaris, as best shown in B. archaica (Plate 1, fig. 1), 

 there is a single column of interambulacral plates in each area extending from just above the 

 second row of ambulacral plates of the peristome to the apical disc, where in this species they 

 are completely covered by the large ocular plates and do not reach the genitals. The full details 

 of this and other cases given are considered under the description of the species. This is the 

 simplest and, I think, the most primitive condition known in any adult echinoid. In my 

 paper on Echini (1896, p. 233) I considered Bothriocidaris as representing the archaic radical 

 as regards interambulacral structure from which the interambulacra in all other Echini could 

 be derived. Further evidence and stutly confirm this view. 



Mr. A. Agassiz (1904, p. SO) says, "The existence of an interambulacral zone composed 

 of a single row of plates [in Bothriocidaris] does not give us any clue to the mode of formation 

 of the Palaechinid type of interambulacra with its manifold rows of plates." I cannot agree 

 with this view, as it seems that the single primordial plate found so universally in j'^oung Echini, 

 or in adults when resorption has not removed it, may fairly be considered as representing in 

 development a stage consisting of a single column which in all but Bothriocidaris is succeeded 

 by additional columns during development, as shown in my first paper and in numerous cases 



