ECHINOIDEA. 



233 



of the apical disc ; but this structure is only composed of the 

 normal ten plates, and wants the supernumerary or " sur- 

 anal " plate of the Saleniadce. 



In the family of the Saleniadce 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 provided with two rows of large tubercles, 

 with crenulated bosses, which may or may not be perforated. 

 The leading character of the family, however, is to be found 

 in the apical disc (figs. 123 and 127), which is of unusually 

 large size, and possesses a super- 

 numerary or " suranal " plate in 

 addition to the ten normal plates. 

 This suranal plate (fig. 127, s) is 

 placed in front of the anus, and 

 it may be single, or it may be 

 broken up into several (not more 

 than eight) elements. Of the 

 genera of the Saleniadce, Acrosa- 

 lenia is essentially Jurassic ; Pel- 

 tastes and Goniophorus are Cre- 

 taceous ; and Salenia itself (fig. 

 123), commencing in the Creta- 

 ceous, is not only found in the 

 Tertiary rocks, but has now been 

 detected in a living condition. 



Lastly, in the great family of the Echinidce the test is 

 usually globular or hemispherical ; and the ambulacral areas 

 are comparatively wide, and always carry two or more rows 

 of tubercles. The interambulacral areas are wide, and 

 carry primary tubercles, which are always imperforate, and 

 are never of very large size. The spines are short and awl- 

 shaped, and their surface is marked with fine longitudinal 

 lines. Of the genera of this family, Glypticus, Magnotia, 

 Polycyphus, the great group termed Stomechinus, and others, 

 occur in the Jurassic ; Codiopsis is Cretaceous ; and Tern- 

 nechinus is Tertiary and Kecent. 



II. ECHINOTHUIIIDJS. In this small but highly remark- 



Fig. 127. Apical disc of Peltastcs 

 Wrighlii, one of the Sakniadce. a, 

 Anus ; g, One of the genital plates ; 

 o, One of the ocular plates ; s, Suranal 

 plate. Twice the natural size. Cre- 

 taceous (Lower Greensand). (After 

 Wright.) 



