ECHINOIDEA. II. r-. 



St. II (64' 34' Lat. N. 31° 12' Long. W. 1300 fathoiiis i°6 C. Bottom temp.) 2 specimens. 



— 76 (60° 50' — 26° 50' — 806 — 4°i — — ) 2 — 



— 8x (61° 44' — 27° oo' — 485 — 6''i — — ) 2 — (younjjl 



— 83 (62° 25' - 28° 30' - 912 - 3°5 - _ , 3 _ 



One young specimen was furtlier taken by the Thor 1904 at 61'' 15' Lat. X., 9° 35' Long. W. 

 goo Meter. 



This species is evidentl)- nearly related to the Californian species Plcxechinus cinctus A. Ag. 

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

 terior end of the test, the actinai 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 //irsii/iis. If it proves 

 to be a constant feature in P. cinctus that onl\- three pairs of plates are in contact with the periproct 

 whereas in liirsiifiis four pairs are so, this will be a very good distinguishing character. (In the I'l. 

 Xordcitskjoldi, to be described in the Report on the Echinoidea of tlie Swedish vSoutli Polar E-xpedi- 

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

 supposed to show more important differences. 



The genus PIcxrchimts is placed among the Pourtalesise b)- Agassiz, mainly on account of 

 its anal snout and the position of the i^eri^Droct; probably also other features: the elongated shape, 

 the apical system, the disjointed sternum and tlie rudimentar\' ph\llodes 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 Urrcliinidcr. It differs essen- 

 tially from the Pourtalesia; 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 Pourtalesi;e it is paired — in Plexecliinus 

 it is single. Further Plcxcclujnis agrees with the Urcchinidcc in regard to the pedicellarite: globi- 

 ferous pedicellarise occur, but no rostrate; the ophicephalous pedicellarise are of the t\pe found in 

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

 iCysfechimis clypcatus (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 ph\llodes are not so very rudimentar\-, at any rate not in P. Iiirsutus, 

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

 Sternopafagus 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 ior ^\a.cu\<^ Plcxec/iimcs 

 among the Urcchi/iidcr, all the Pourtalesise, except Ster)iopatagus. which Agassiz will even refer to 

 the Urechinids (without sufficient reason, as far I €an 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 



Tlie Ingolf-Expedition. I\'. .;. 8 



