ECHINOIDEA. I. 



69 



Agassiz ( Blake -Echini. Pl.XV) has figured several stages of development of this species. 

 As among the material of Ph. placenta collected by the Ingolf several small specimens arc found, 

 especially from st. 25 (the Davis Strait), 1 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 very well 

 with what is shown by the specimens before me. Whether this is due to the fact that the figures 

 given by Agassiz are inaccurately 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 Gulf of Mexico, which our museum 

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

 I shall only figure a couple of stages of the development of the apical area in the specimens in hand. 



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

 PI. XV. Fig. 3 figures the apical area of a specimen of a diameter of8 mra . The difference between these 

 two figures is rather conspicuous. In the specimen figured here the ocular plates have a peculiar, 

 spade -like form, and the genital plates almost join inside of them, so that the ocular plates 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 plates is quite different, and the ocular plates reach far inside 

 of the genital plates. On PI. IV. Fig. 1 the apical area of a specimen of a diameter of 37 mm is figured. 

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

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

 vounger 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 thev are developmental stages of the same species. The peculiar small, oblong plates in the skin 

 of the region round the anal opening, begin already to appear in specimens of a diameter of i5 mm . 

 (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 mm 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 (Pl.XV. Fig. 5) figures the apical area of a Ph. placenta 

 of a diameter of 17""; this figure agrees as badly with a specimen of i7 mm 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 fact that the 

 specimens before me are really Phorvwsoma 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 plates distinctly. 



A large material of this species has been obtained by the Ingolf -Expedition on the following 

 stations : 



St. 24. (63° 06' N. Lat, 56° oo' W. L. 1190 fins. Mud. 2 7 bottom temp.). 1 specimen. 



)• 167 

 ). 27 



)• 5 - 

 )• 2 - 



