ECHINOIDEA. I. 



69 



Agassiz (; Blake -Echini. PI. XV) has figured several stages of development of tliis species. 

 As among the niaterial of Ph. placenta collected by tlie <- Ingolf » several small specimens are foitnd, 

 especially from st. 25 (the Davis Strait), I have been able to follow the development of the apical area, 

 and have found that the description of this development given by Agassiz does not agree verv well 

 with what is shown by the specimens before me. Whether this is dne to the faet that the figures 

 given by Agassiz are inaccnrately drawn, or perhaps a confounding with another species has taken 

 place, I shall not try to decide. (The possibility of the West-Indian specimens of Ph. placenta being a 

 special local form, seems to be excluded: some specimens from the Gnlf of Mexico, whicli our mnsenm 

 has received from the Smithsonian Institution, agree exactly with those taken in the Davis Strait.) 

 I shall onh- figure a couple of stages of the development of the apical area in the specimens in hånd. 



On PI. IV. Fig. 2 the apical area of a specimen of a diameter of 7""" is figured. Agassiz on 

 PI. XV. Fig. 3 figures the apical area of a specimen of a diameter of 8™"". The difference between these 

 two figures is rather conspicuous. In the specimen figured here the ocnlar piates have a peculiar, 

 «spade -like form, and the genital piates almost join inside of them, so that the ocular piates only 

 just touch the anal area; the madreporite may already be distinguished. In the figure of Agassiz 

 the form of the ocular and the genital piates is quite different, and the ocular piates reach far inside 

 of the genital piates. On Pi. IV. Fig. i the apical area of a specimen of a diameter of ■yi^^ is figured. 

 The development of small piates, partly at the cost 01 the genital and ocular piates, is here already 

 rather advanced, the ocular piates, however, having still essentially kept the form characteristic of the 

 younger stages. (In the adult animal this form is no more to be recognized.) Even if all possible 

 transitional stages between the two figured here were not found, there could scarcely be any doubt 

 that they are developmental stages of the same species. The peculiar small, oblong piates in the skin 

 of the region round the anal opening, begin already to appear in specimens of a diameter of 15""°. 

 (They have here been drawn a little too regular.) Agassiz (PI. XV. Figs. 9 and 11) figures the apical 

 area of specimens of a respective diameter of 28""" and 41'""". The resemblance to the figures given 

 here is not striking; but the figures are rather indistinct, so that it is difficult to compare the details 

 of the two sets of figures. Further Agassiz (PI. XV. Fig. 5) figures the apical area of a Ph. placenta 

 of a diameter of 17™'"; this figure agrees as badh- with a specimen of i7ni'" now before me, as does 

 the figure 3 of Agassiz with the apical area of a specimen of 7""" figured here. — A comparison of 

 these two figures in Agassiz (Figs. 3 and 5) conveys the direct impression that they do not belong 

 to one species. But whatever the case may be with regard to these figures, it is a sure faet that the 

 specimens before me are really Phormosonia placenta. It is still to be observed that the figures given 

 here have been drawn from dried specimens; in specimens in spirit it is generally impossible to see 

 the limits between the piates distinctly. 



A large material of this sj^ecies has been obtained by the < Ingolf -Expedition on the foUowiug 

 stations : 



St. 24. (63° 06' N. Lat, 56° 00' W. L. 1 190 fms. Mud. 2° 7 bottom temp.). i specimen. 



- 25. (63^ 30' - 54' 25' - 582 - - 3° 6 - ). 167 - 

 -28. (65° 17' - 55° 42' _ 420- - 3-8 - ). 27 - 



- 40. (62° 00' — 21° 36' — 845 — - 3^9 — ). 5 — 



- 63. (62=40' — 19° 05' — 800 — — 4" 3 — ). 2 — 



