SYSTEMATIC CLASSIFICATION OF ECHINI. 221 



each area or more than two up to twenty, but with an equal number in each half-ambulacrum. 

 The primordial interambulacral plates may be retained in the basicoronal row, or maj^ be re- 

 sorbed in the advance of the peristome. There are three or more, usually more, columns of 

 plates in each interambulacral area, and there may be as many as fourteen, the highest number 

 at present known. The plates may be imbricate or not, but when imbricate, the ambulacrals 

 imbricate adorally and the interambulacrals aborally and from the center laterally and over 

 the ambulacrals (text-figs. 32-38, p. 75). The primordial ambulacral plates are situated on 

 the peristome around the mouth as in Cidaris, as known in some types and inferred in others. 

 Succeeding these on the peristome there are many rows of ambulacral plates only, extending 

 to the corona, or in addition there may be non-ambulacral plates in certain genera. In the 

 Echinocystoida the apical disc is doubtful, but in the Perischoechinoida it is small proportion- 

 ately to the diameter of the test, a progressive character (Plate 55, figs. 2, 3). Oculars are 

 typically all insert, but rarely one or more to all may be exsert. Ocular pores are rarely visible. 

 Genital plates are of moderate size and have from two to ten or eleven pores each. In one 

 species only (Lepidechinus tessellatus, Plate 63, figs. 7, 8) there is a single genital pore in a plate 

 as far as preserved. A madreporite is only occasionally recognizable. The periproct where 

 known is covered with many thick plates as in Cidaris, and no suranal is recognizable. The 

 lantern is inclined (Plate 27, figs. 4-6) like that of the young of modern regular Echini, and 

 as in that group, is composed of forty pieces. The teeth are grooved as in the Cidaroida and 

 Aulodonta. The epiphyses are narrow; brace and compass as in modern forms. The foramen 

 magnum is moderately deep and the lateral wings of the pyramids have ridges for the 

 attachment of interpyramidal muscles which were relatively long. The dorsal face of the pyra- 

 mids is smooth as in the Cidaroida, not pitted as in the Centrechinoida. No perignathic 

 girdle is known and lantern muscles were apparently attached directly to the base of the inter- 

 ambulacral plates (text-fig. 221, p. 193), as in young Cidaris. There may be primary spines 

 with secondaries, and the tubercles of primary spines perforate; or there may be secondary 

 spines only, the tubercles of which are imperforate. Respiration was quite likely effected by 

 Stewart's organs, as in the Cidaroida. There certainly were no peristomal gills as in the Cen- 

 trechinoida. Sphaeridia are unknown. 



The Echinocystoida as an order is characterized by the fact that the anus was apparently 

 eccentric in an interambulacrum. The material is not well preserved and is imperfectly known. 

 There are two or four columns of ambulacral and eight or nine columns of interambulacral 

 plates in each area respectively. The plates are thin and imbricating. What the relations 

 of this group are is uncertain, though it has certain approximations to the Lepidocentridae 

 and may be an offshoot from a common early stock. As an irregular group in the Palaeozoic 

 it is considered first in order to dispose of it (p. 250). 



The Palaeodiscidae as a family is the least specialized of its order as there are only two 



