REGULAR EUECHINOIDS. 379 



Of the genera of this family, Cidaris itself (fig. 249) is the most im- 

 portant, ranging from the Trias to the present day. Rhabdocidaris is 

 Jurassic and Cretaceous ; Diplocidaris is Jurassic ; and the Porocidaris 

 of the Secondary and Tertiary periods has now been detected by Sir 

 Wyville Thomson in a living condition. In the aberrant Cretaceous 

 genus Tetracidaris, the interambulacral zones are composed each of four 

 rows of plates, reduced to two rows in the neighbourhood of the apex. 



The genus Eocidaris, based upon a form which occurs in the Permian 

 rocks of Germany, has been generally placed among the Palechinoids, in 

 the vicinity of Archozocidaris. According to the recent researches of 

 Kolesch, however, the interambulacral and ambulacral areas in E. Key- 

 serlingi are two-rowed, and the genus is therefore properly referable to 

 the Euechinoids. The Devonian and Carboniferous Urchins which have 

 been referred to Eocidaris have more than twenty rows of plates in the 

 corona, and must therefore be removed from this genus as now under- 

 stood. With our present knowledge, therefore, the Permian Eocidaris 

 must be regarded as the oldest type of the Euechinoids. 



Family 2. Salenidaz. — In this family the test is generally spher- 

 oidal, hemispherical, or depressed, and the ambulacral areas are 

 always narrow, sometimes straight, sometimes flexuous, and without 

 large primary tubercles. The interambulacral areas are always pro- 

 vided with two rows of large tubercles, 

 with crenulated bosses, which may or 

 may not be perforated. The lead- 

 ing character of the family, however, 

 is to be found in the apical disc (figs. 

 250 and 251), which is of unusually 

 large size, and possesses a supernum- 

 erary or " suranal " plate in addition 

 to the ten normal plates. This sur- 

 anal plate (fig. 250, s) is placed in 

 front of the anus, and it may be 

 single, or it may be represented by Fig. 250. — Apical disc of Peitastes 



-, z , •■ . , , \ , Wrightii, one of the Salenidce. a, 



Several (not more than eight) ele- Anus; g, One of the genital plates; 



ments. From a morphological point %£* &£ ^ n^S ; ,fc. Sui S£ 



Of View the SUranal plate may be teceous (Lower Greensand). (After 



compared with the " dorso-central " 



plate of the Crinoids, while the genital and ocular plates respectively 

 correspond to the basals and radials of the Crinoidal calyx. The 

 " madreporite " is imperfectly developed, and often hardly recog- 

 nisable. The peristomial membrane is furnished with calcareous 

 plates which differ from the peristomial plates of the Cidaridaz in 

 the fact that the rows of ambulacral pores are not continued over 

 them to the mouth. 



The genus Salenia itself (fig. 251) appears for the first time in the 

 Cretaceous rocks, and still survives at the present day. Peitastes, with 

 a similar range in time to Salenia, differs from the latter in the fact that 



