!8o ECHNIOIDEA. II. 



shown by Ad. S. Jensen 1 that in postglacial time Greenland has had a period of milder climate, when 

 forms snch as Zirphcea crispata occurred along the Greenland Coast; this bivalve now has its Nor- 

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

 nius parma. During that milder period the extension of this species along the Northern Coast of 

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

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

 has been undertaken, it is impossible to state anything more definitely about this question. 



Echinus csculcntus also occurs at Spitsbergen, according to L, tit ken. 2 This statement is reg- 

 arded as very doubtful by Michailovskij.3 In any case this occurrence would not justify counting 

 Ecli. esculentus among the species of the Arctic littoral region. The Gulf Stream still makes itself 

 felt even at the Southwestern end of Spitsbergen, which would account for the presence of this species 

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

 not reach. 



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

 the bottom temperature is negative. It is limited from the deep-sea of the Atlantic South of Iceland by 

 the three submarine 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 Jeffrey si. It is true that Eclii>ms Alexandri and Spatangus Raschi have 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 Jeffrcysi has been recorded several times from the Atlantic, both the European and 

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

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

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

 bottom temperature is positive. 



Pourtalcsia Jeffrcysi is nearest related to P. Wandeli, the species widely distributed in the 

 warm area of the Atlantic Deep Sea; it may be said with certainty that P. Jeffrcysi has beeu devel- 

 oped from a form very much like this species (perhaps the ancestor of both P. Wandeli and Jeffrcysi), 

 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 physical conditions had become 

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



1 Ad. S. Jensen. On the Mollusca of East Greenland. I. Lamellibranchiata. With an Introduction on Greenlands 

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



2 Chr. Liitken. Et Bidrag til Kundskab om Spitzbergeus Echinoderm-Fauna. (Vidensk. Medd. Naturh. Foren. 

 Kobenhavn. 1S71. p. 305.) 



3 M. Michaiiovsklj. Zoologische Ergebnisse der Russischeu Expedition nach Spitsbergen. Echiuodermeu. (Ann. 

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



