KONGL SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 95 



the significance of Avhich perhaps wilLbe fouiid, when the researches are brought to 

 a close that are just begun, on the gases contained in the abyssal waters. 



Urechinus, Cystechinus and Calymne, Abyssal, Ethmophract, Meridosternous, and 

 true meinbers of the ancient group of Adetes, are most decidedly apetalous. Are there, 

 atnong the genera of that group, any forms extant that, from the structnre of their dorsal 

 ambulacra, may be put forth as the littoral or sub-littoral main stock, of which Urech- 

 inus and its living allies may be the deep-sea representatives? Anancites and Offaster 

 had the paired ambulacra level with the perisome. On the flanks the pores have their 

 perforations close together, rather diagonal, a little below the centre; in the dorsal 

 portion the crowded plates, at least twice as broad as they are long, with the per- 

 forations transverse, larger, separated, nearer to the outer and the adoral margins, 

 make very open and soniewhat rudimentary petals, that evidently were the sites of 

 branchial leaflets. Hemipneustes, combining v/ith the ancient character of an exquisite 

 zig-zag sternum an impressed III bearing peculiar peripodia, a produced and laterally 

 expanding labrura, a madreporite widely spreading within the strictly ethmophract 

 calyx ^), has its paired ambulacra semi-petaloid, the pores of I a, V b, II a, IV h, 

 being large, transverse, with the outer perforation a slit nearing the posterior margin, 

 while the pores of I 6, V a, II b, IV a decrease dorsally, as the former increase, and 

 become very minute and sub-median, finally diagonal. Holaster, also, has the I, V, II, 

 IV, semi-petaloid, but the pores of the anterior series not nearly so minute relatively, 

 and diagonal only very near the calyx. In Cardiaster the petala are almost completelj^ 

 developed, the anterior pores somewhat less, but transverse and evidently branchial. 



On the analogy thus clearly offered by the Prymnadete and Prymnodesmian 

 Spatangi, the conclusion to be drawn from this difference, in the structure of their 

 ambulacra, between the recent abyssal and those ancient Meridosterni, seems lawfully 

 to be, that these latter, Anancites, Offaster, Hemipneustes, Cardiaster, and others, among 

 which at least Anancites and Offaster lived in a polythalamian sediment comparable 

 to that of the great depths of the actual seas, were not truly abyssal, but inhabitants 

 of less deep, though oceanic parts of the Mesozoic sea. If they still survive in generi- 

 cally allied or altered forms, these are to be looked for on the sloping bottora betAveen 

 the littoral and continental shelves and the great depths. But Avhether extinct or still 

 living, they once had or yet have, in Urechinus, Cystechinus, Calymne, or in ancestors 

 of these, their representatives in the abysses of the ancient ocean, Avhich are those also 

 of our days. 



It may be that the knoAvn species of the Pourtalesiada^, Avith apetalous ambu- 

 lacra, are the abyssal representatives of other members of their family provided Avith 

 developed petaloid branchial apparatus, and living nearer to the light and the air, in 

 the littoral zone or in the \'ast regions interjacent betAveen that and the great depths. 

 In a fossil state such forms may possibly one day be met with, in Cretaceous layers 

 enclosing Anancites and Echinothuria. 



1) See woorlcut, p. 70. 



