80 THE VOYAGE OF H.M.S. CHALLENGER. 



The abactinal system of Melonites, on the contrary, from the drawings of Eoemer and of 

 Meek and Worthen, and of Palceechinus from those of Bailey, resembled far more that of 

 the Cidaridse proper than of the recent Echinothurid^. The ocular and genital plates are 

 comparatively large, in striking contrast to the adjoining minute ambulacral and inter- 

 ambulacral plates of the abactinal region of the test, forming a most distinct line of 

 division between the so-called coronal plates and the plates of the abactinal system. 

 This is somewhat remarkable, as the plates of the abactinal system are evidently, from 

 what we know of their appearance in young Echinids, developed comparatively later 

 than the coronal plates. 



In Lepidocentrus and in ArcluBOcidaris, on the contrary, the plates of the abactinal 

 system resemble far more in their size and arrangement those of the abactinal system 

 of the recent Echinothuridse. In Lepidocentrus, for instance, the genital plates bear about 

 the same proportion to the plates of the anal system which we find in some of the recent 

 Echinothuridse, and there exists at the apical extremity of the coronal plates the same 

 difficulty in defining where the interambulacral plates terminate and the plates of the 

 apical system begin. 



The existence of double pores in the apical system does not seem to indicate in the 

 Palseechinidas an abnormal structure among Echinoidea, Loven has already shown how in 

 the Arbaciadse the ocular pore is divided into two, and we must remember that in the 

 Spatangoids it is the contrary process which takes place, in the passage of double pores 

 to simple pores and then to double pores again between the actinostome and the 

 abactinal system. It seems to me to be merely the first indication of the subsequent 

 subdivision of the ambulacral pores to be traced among the Echinoidea which becomes so 

 universal among the regular Echinids, though it is transferred to a difi'erent part of the 

 poriferous zone, while in many Spatangoids it is limited to special portions of the 

 poriferous zone. In the Palseechinidse we may consider this the first indication probably 

 of the specialisation of any one of the ambulacral tentacles. 



In a specimen of ArclicBOcidaris from the Keokuk Limestone showing the actinal side 

 of the test, the actinal plates immediately adjoining the actinostome are small imbricat- 

 ing plates, forming a somewhat indistinct line of demarcation with the true coronal plates 

 quite as clearly defined as in the recent Echinothuridse when compared to the primary 

 coronal plates siirmouuted by a single large primary tubercle as in Phorr)iosoma. The 

 ambulacral areas expand at the line of junction with the actinal membrane, and the rows 

 of pores are more distinct than in the poriferous zones of the corona. The interambu- 

 lacral plates are quite small ; but both the ambulacral and interambulacral plates ex- 

 tend to the very centre of the actinostome, where the actinal membrane is attached to the 

 outer edge of the teeth, There appear at the line of junction between the actinal plates 

 and the primary interambulacral plates interstices corresponding in position to those 

 which give passage to the gills in the recent Echinothuridse on each side of the poriferous 



