8<1 SVKN LOVEN, ON THE ECHINOIDEA BESCRIBED BY LINN^US. 



bv means of a new inodns operandi of the formativo ac- 



tivity. 



In tbis genus the formation of spiue-bearing tubercles, 

 its gradual progress from below ixpwards, its retarded appea- 

 ranee or even failing on tbe disks, is to a great extcnt depen- 

 dcnt upon another of tbe skeletal elements early attaining a 

 deffree of development rarely seen among tbe Ecbinoids. 



Wben tbe soft tissiies are removed from tbe test of an 

 adult Arbacia tbere comes into view, on tbe calycinal system, 

 on the interradia as well as on tbe ambiilacra, tbeir areolas 

 and zones of pores, a growtb superposed to the proper surface 

 of the plates, luxuriant in some, evanescent in a few species, 

 of ealcified protuberances not serviug as supports to organs 

 of any kind, Tab. 7,fig. 1 — 4; Tab. 8,jig. 4—8. Thougb by no 

 means peculiar to tbis genus, being in ftict of extensive, pre- 

 sumablv universal occurrence among Ecbinids, it is developed 

 in the Arbaciae in a remarkable manner and afFords specilic 

 characters of no little value. It is not unlikely that Linn^us 

 designed it by tbe word :-)punctatis: of the diagnosis. It bas 

 commonly been comprehended under tbe general name of 

 »granulation», »granules>^, ->miliaries;', terms including at tbe 

 same time true spine-bearing tubercles, secondary and tertiary, 

 as well as those of the pedicellarife and the spherids. It con- 

 stitutes, bowever, a system by itself, higbly dissimilar and 

 even antagonistic to that of the tubercles, and it will there- 

 före be convenient, at present and in order to keep it distinct, 

 to designate it by a proper name, for which I propose that 

 of epistroma. In the adult Arbacia it generally appears under 

 two distinct forms: partlv as a continuous linely striated laycr, 

 compact or nearly so, in which are imbeddcd radiating rows 

 of verv minute glossy knobs, and partly as assemblages of 

 larger, separate but crowded, sessile or sub-pedunculated pro- 

 tuberances; Tab. 7, fig. 1 — -4. Wherever tbe test is devoid of 

 spine-bearing tubercles, tbe former structure prevails, as on 

 the calycinal system and on the disks of some species, jig. 3, 4 ; 

 the latter makes its appearance in tbe vicinity of tubercles, 

 that is in the ambulacral areola and on the greater portion or on 

 the sides of each interradium, fig. 1, and on the zones, Tab. 8, 

 fig. 5. Consequently, whcu the epistroma is followed from below 

 upwards, it is seen that on the basis of the test, whcre the 

 tubercles are closely crowded, it is much less developed thau 



