j8o ECHnioidea. II. 



shown by Ad. S. Jensen' that in pcstglacial time Greenland has had a j^eriod of milder climate, when 

 forms sucli as Zirphæa crispata occnrred along the Greenland Coast; this bivalve now has its Nor- 

 thern limit at the Gnlf of St. Lawrence — as seems also to be the case with the Atlantic Echinarach- 

 iiiiis partiia. During that milder jDcriod the extension of this S2:>ecies along the Northern Coast of 

 America from the Pacific to the Atlantic mav have taken place (— or it mav even have taken place 

 before the Ice Period — ). Until a carefnl zoological exploration of the waters to the North of America 

 has been nndertaken, it is impossible to state anything more definitely about this qnestion. 



Echinus esciikntus also occurs at Spitsbergen, according to Iviitken.^ This statement is reg- 

 arded as very doubtfnl by Michailovskij.'i In any case this occurrence wonld not justify connting 

 Ech. csculcntus among the species of the Arctic littoral region. The Gnlf Stream still niakes itself 

 felt even at the Sonthwestern end of Spitsbergen, which would acconnt for the presence of this species 

 here. It will certainly not be found at the East and North Coast, to which the Gnlf Stream does 

 not reach. 



The Arctic abyssal region comprises the deep basin of the sea to the North of Iceland, vvhere 

 the bottom temj^eratnre is negative. It is limited from the deep-sea of the Atlantic Sonth of Iceland by 

 the three snbmarine ridges: one passing over the Denmark Strait from Iceland to Greenland, another 

 from Iceland to the Faroe Islands and the third from the Faroe Islands to the Hebrides. The north- 

 ern limits of this region are still unknown. — Only one species of Echini occurs in this region, viz. 

 Pourtalcsia Jeffreysi. It is true that Ec/ii/nis AIcxandri awå Spatangus Raschi hawe been recorded a 

 single time each from a considerable depth and negative bottom temperature off Norway; but these 

 cases are undoubtedly quite exceptional, the former species decidedly belonging to the Atlantic deep- 

 sea Fauna, the latter to the boreal and the Atlantic Fauna. 



Pourtalcsia [cffrcysi has been recorded several times from the Atlantic, both the European and 

 the American side, but, as has been shown above, this is due to a confusion with the nearly related 

 Pourt. Wandeli. In reality Poiirt. Jeffreysi is known only from the arctic abyssal region. Its bathy- 

 metrical extension is rather great, from 125 — 1300 fathoms; but it scarcel)' ever occurs where the 

 bottom temperature is positive. 



Pourtalesia Jeffreysi is nearest related to P. Wandeli, the species widely distributed in the 

 «warni area of the Atlantic Deep Sea; it may be said with certaint\- that P. Jeffreysi has been devel- 

 oped from a form very much like this species (perhaps the ancestor of both P. Wandeli and Jeff'reysi\ 

 which was probably distributed over the whole of the Northern Atlantic, thus north of the ridges also, 

 at a time when a more uniform climate prevailed there. When the recent conditions developed the speci- 

 mens to the North of the ridges were thus isolated and developed into a separate species. Or perhaps 

 the ancestor of the species wandered into the northern region, after its plnsical conditions had become 

 like those now prevailiug there. This, of course, caunot be decided; but in any case P. Jeffreysi was 



i Ad. S. Jensen. On the MoUusca of East Greenland. I. Lamellibranchiata. With an Introductiou on Greenlauds 

 fossil Mollusc-Fauna from the quateruary time. Meddelelser om Gronland. Vol. XXIX. 1905. 



= Chr. Liltken. Et Bidrag til Kundskab om Spitzbergeus Echinoderm-Fauna. (Vidensk. Medd. Naturh. Poren. 

 Kobeuhavn. 1S71. p. 305. 1 



3 M. Michailovsklj. Zoologische Ergebnisse der Russischen Expedition uach Spitsbergen. Echiuodermeu. (.-Vun. 

 Mus. Zool. de l'Acad. Inip. St. Petersbourg. VII. 1902.) 



