ECHINOIDEA. II. 



57 



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



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



— 81 (61° 44' — 27° 00' — 485—6°! — — ) 2 — (young) 



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



One young specinien was further taken b\" the Thor 1904 at 61° 15' Lat. N., 9° 35' Long. W. 

 9CX3 Meter. 



This specie.s is evidently nearly related to the Californiaii species Plrxrchiinis ciiictiis A. Ag. 

 It is, however, easih- distinguished from the latter species by the very different outHne 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 hirsutiis. If it proves 

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

 whereas in hirsiifus four pairs are so, this will be a ver>' good distinguishing character. (In the PL 

 Xonffiisk/uldi, to be described in the Report on the Echinoidea of the Svvedish South Polar Expedi- 

 tion, only three pairs of piates are in contact with the periproct). — The pedicellariæ can scarceh' be 

 supposed to show more important differences. 



The genus Plcxcchinits is placed among the Pourtalesiæ b\' Agassiz, niainly on account of 

 its anal snout and the position of the periproct; probabh' 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 evidenth- more nearly related to the Urcclihiida-. It differs essen- 

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

 most prominent characters of the Pourtalesiæ 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 Pourtalesiæ it is paired — in Plcxcchiints 

 it is single. Further P/exrchiiiits agrees with the Urccliinidcc in regard to the pedicellariæ: globi- 

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

 Urechinus (the elongate form of ophicephalous pedicellariæ characteristic of Pourtalesiæ is found in 

 vCystrcIiiiins clypeafns' (the thick-plated form), but it is not certain that this is an Urechinid, the struc- 

 ture of its test being quite insufficientl\- known). Also the structure of the spines points towards the 

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

 seem to me less important. The phyllodes are not so ver\- rudimentary, at any rate not in P. liirsntus. 

 in which the two or e\'en three inner tubefeet in each series are distinctly penicillate; the faet that 

 Sternopatagiis has j^enicillate tubefeet, liowever, 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 Plexechinns 

 among the Urcchi)iidæ, all the Pourtalesiæ, except Sternopatagiis. which Agassiz will even refer to 

 the Urechinids (without sufficient reason, as far I can see (comp. below)), having onl\ 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 di.sjointed. Regarding the sternum both 



The Ingotf-Expedition. [V. 2. 8 



