ECHINOIDEA. II. en 



St. ii (64 34'Lat.N. 31 12' Long. W. [300 fathoms r°6 C. Bottom temp.) 2 specimens. 



7(1(60° 50' 26° 50' 806 1 1 — )2 



— 81 (6i°44' 27°c»' 485 6° 1 )2 (young) 



-83 (62° 25' - 28° 30' 912 3° 5 - ) 3 



One young specimen was further taken by the Tlior 1904 at 61 15' Lat N., 9° 35' Long. W. 

 900 Meter. 



This species is evidently nearly related to the Califoruian species Plexechinus cinctus A. Ag. 

 It is, however, easily distinguished from the latter species by the very different outline of the pos- 

 terior end of the test, the actinal keel being much higher and the anal snout much less prominent in 

 the Atlantic than in the Californian species; the periproct is also more sunken in hirsutus. If it proves 

 to be a constant feature in P. ductus that only three pairs of plates are in contact with the periproct 

 whereas in hirsutus four pairs are so, this will be a very good distinguishing character. (In the PL 

 Nordenskjoldi, to be described in the Report on the Echinoidea of the Swedish South Polar Expedi- 

 tion, only three pairs of plates are in contact with the periproct). — The pedicellarise can scarcely be 

 supposed to show more important differences. 



The genus Plexechinus is placed among the Pourtalesise by Agassiz, mainly on account of 

 its anal snout and the position of the periproct; probably also other features: the elongated shape, 

 the apical system, the disjointed sternum and the rudimentary phyllodes are taken as arguments in 

 favour of such a position of the genus though it is not stated clearly. The genus certainly shows 

 some Pourtalesian affinities, but it is evidently more nearly related to the Urechinidce. It differs essen- 

 tially from the Pourtalesise and agrees with the Urechinids in having a flat peristome, one of the 

 most prominent characters of the Pourtalesise being the vertical peristome at the inner end of a deep 

 groove. Another feature of eminent importance is the structure of the anterior paired interambulacra; 

 the second plate is single in the Urechinids, whereas in all Pourtalesise it is paired — in Plexechinus 

 it is single. Further Plexechinus agrees with the Urechinidce in regard to the pedicellarise: globi- 

 ferous pedicellarise occur, but no rostrate; the ophicephalous pedicellarise are of the type found in 

 Urechinus (the elongate form of ophicephalous pedicellarise characteristic of Pourtalesise is found in 

 «Cystechimis clypeatus* (the thick-plated form), but it is not certain that this is an Urechinid, the struc- 

 ture of its test being quite insufficiently known). Also the structure of the spines points towards the 

 Urechinid affinity. On the other hand several of the characters pointed out by Professor Agassiz 

 seem to me less important. The phyllodes are not so very rudimentary, at any rate not in P. hirsutus, 

 in which the two or even three inner tubefeet in each series are distinctly penicillate; the fact that 

 Stcniopatagus has penicillate tubefeet, however, shows that much stress cannot be laid on this feature. 

 If it were of greater importance it could, of course, only be a further argument for placing Plexechinus 

 among the Urechinidce, all the Pourtalesise, except Sfernopatagtis, which Agassiz will even refer to 

 the Urechinids (without sufficient reason, as far I can see (comp. below)), having only simple tubefeet. 

 The apical system shows so great differences in the whole Ananchytid group that it seems unreason- 

 able to lay much stress on its being a little more or less disjointed. Regarding the sternum both 



Thf* Ingolf-Expedition. IV. 2. 8 



