TERMINOLOGY. 27 



of adjacent plates in the several succeeding rows. In the figures, the columns of interambula- 

 cral plates are numbered from one upward (Plate 57, fig. 1; text-fig. 246). 



Adradial suture, a term introduced by Bather (1909a), is the suture between the interam- 

 bulacrum and the ambulacrum. As a corollary, in Palaeozoic Echini, when there are more 

 than two columns of interambulacral plates, those lying next the ambulacra are called the 

 adradial plates. The term adambulacral plates or columns has been used, but the term adradial 

 has the advantage of brevity. 



Median suture is the suture in the median line of the ambulacrum. When there are only 

 two vertical columns of plates, it is perfectly obvious, but when more, it is distinguishable 

 by the size of the two medial columns, as in Melonechinus (Plate 56, fig. 4) ; or if the plates 

 do not differ in size, it is distinguished by the fact that the pores in the plates are set on the 

 side toward the next adjacent interambulacrum and therefore are set on opposite sides of the 

 plates on either side of the median sulure, Lepidesthes wortheni (Plate 67, fig. 8). The term 

 median suture in Recent Echini is ordinarily applied also to the interambulacrum, and in Recent 

 foims it is applicable, as there are two columns of plates; but, taking sea-urchins as a whole, 

 it is not applicable to the interambulacrum; for it is only a coincidence of ordinal or specific 

 character, if there is an even number of columns, and when there is, as in Palaeechinus quadri- 

 seriaHs (Plate 30, fig. 3), the suture in the median line is not distinguishable from any other 

 interambulacral suture except the adradial. 



In considering ambulacral plates, a certain degree of nomenclature is convenient. Pri- 

 mary plates extend from the middle of the ambulacrum to the interambulacral suture (Palae- 

 echinus, text-fig. 9). Demi-plates extend from the interambulacrum inward, but do not reach 

 the middle of the ambulacrum. Occluded plates extend outward from the middle of the am- 

 bulacrum, but do not reach the interambulacrum. Demi- and occluded plates are typically 

 shown in Lovenechinus (text-fig. 11). Isolated plates do not reach either the interambulacral 

 suture or the middle of the ambulacrum; such plates are extensively developed in Melonechinus 

 and Lepidesthes (text-figs. 13, 14, p. 54). This nomenclature was based on the elements of 

 compound plates in the Centrechinoida,' but it seems that it may equally be applied to the 

 complex arrangement of simple plates in Palaeozoic Echini. 



' The name Diadema, given by Schynvoet in 1711, cannot stand for a genus of echinoderms. The first post-Linnaean 

 use of the generic name Diadema is in the "Museum Colonnianum," a dealer's catalogue published in London, in 1797; 

 of this work, the author's name does not appear on the title page, but in the British Museum copy it is stated on two author- 

 ities that the author was G. Humphreys. There are 12 species listed under Diadema, but of these only one is recognizable, 

 as it is stated to be the same as Echinus esculentus Linne. On this evidence, if this work should be accepted, which is very 

 doubtful, the genus Diadema would become a synonym of Echinus, as esculentus is the type of that genus. The next post- 

 Linnaean use of the name Diadema was by Schumacher, 1817, who gave this as a generic name for a cirriped, basing it on 

 Lepas diadvma Linn6, which Schumacher (1817, p. 90-91) changed to Diadema vulgaris. This name is a synonym of Coronula 

 diadema Oken (Lehrb. Naturg., 1815, part I, section 1, p. 3G0). Ranzani, in 1820, also used the generic name Diadema for 

 a cirriped, adopting, as did Schumacher, Lepas diadi-imi Linn6, as the type and giving it the new name Diadema candidum. 



