TERMINOLOGY. " 29 



The term peristome is not used in Lov6n's sense, l)ut in the more usual sense of the buccal 

 membrane with its plates extending from the mouth to the basicoronal plates. The peristome 

 may be more or less plated with ambulacral plates only, with ambulacral and non-ambulacral 

 plates, with non-ambulacral plates only, or may he a naked membrane (text-figs. 40-54, p. 80). 

 Primordial ambulacral plates: these are the first formed ambulacral plates, ten in number, 

 primarily situated around the mouth in the buccal membrane (Plate 3, fig. 11), or secondarily as 

 basicoronal plates of the corona, as is usual in clypeastroids and spatangoids (Plate 3, fig. 15). 



Primordial interambulacral plates: these are the first formed interambulacral plates, five 

 in number, one in each area, which may be retained at the base of the corona in the adult, or 

 may disappear by resorption (text-figs. 22-31, p. 70). 



Ocular and genital plates are used in preference to radial and basal, as not implying an 

 homology with those plates in crinoids. The sum of these plates may conveniently be termed 

 the oculo-genital ring. 



Gills are external and interradial in position, situated on the border of the peristome, 

 where they may well be called peristomal gills (text-figs. 55, 56, p. 83) ; or they are internal and 

 radial in position, as Stewart's organs, Cidaris; or they may be external as modified tube- 

 feet, when they may be called ambulacral gills to distinguish them from the interradial 

 peristomal gills. 



The terminology applied to spines is necessarily rather arbitrary, as such great difference 

 in size exists, but it is convenient to have some basis for description. Primary spines are 

 the large spines occurring in such types as Cidaris or Arbacia. Secondary spines are best 

 represented by the smaller spines clasping about the base of the large ones in Cidaris. Miliaries 

 are minute spines seen between the smaller ones in Cidaris, or all the spines of Echinarachnius 

 could be classed under this head. These are spines so small that it requires a microscope to 

 see their structure. In the Palaeozoic we find primary spines and secondary spines in Archaeo- 

 cidaris (Plate 9, fig. 13) and Lepidocidaris (Plate 17, figs. 10-14). In some types secondary 

 spines only are known, as in Melonechinus (Plate 52, figs. 10, 11) and allies. Mihary spines 

 are so far known in Archaeocidaris only (Plate 11, fig. 4). The points of attachment of the 

 spines are the tubercles which may be primary, secondary', etc., according to their size. The 

 consideration of nomenclature of the parts of spines, plates, etc., of Echini is very clearly and 

 definitely set forth by Bather (1909a). 



The Aristotle's lantern is described as inclined, when in side view it subtends an angle 

 of about 90°, the character of the Echinocystoida and Perischoechinoida (Plate 27, fig. 6) ; 

 erect, when the faces of opposite sides are approximately parallel, the usual character of Recent 

 regular Echini (Plate 5, fig. 2) ; and procumbent, when the flaring lantern rests almost on the 

 base of the test, as in the Clypeastrina. Terms applied to the parts of the lantern are as follows. 

 There are five teeth which are grooved or keeled (text-figs. 210, 212, p. 184). Each tooth is 



