ECHINOIDEA. I. 



of the actinal side and those of the abactinal side. (Our specimen is exactly in this condition). It 

 does not appear from the habitus figures given by Agassiz and Koehler that it is the same species 

 but as Agassiz only figures the abactinal side, Koehler only the actinal one, the figures do not 

 disprove the identity either. On the other hand, the detail-figures seem to agree, especially with 

 regard to the arrangement of the pores which is rather characteristic. I therefore think it very prob- 

 able that the species of Koehler is really identical with the Ph. Petersii of Agassiz, which latter is 

 accordingly in no way synonymous with :>/%.> uranus Wyv. Thomson. 



This species is distinguished hy the following characters. The tube feet are placed on the 

 actinal side in one almost regular series, on the abactinal side they are placed in three series very 

 close together. The spicules are irregular fenestrated plates that do not seem to be arranged in longi- 

 tudinal series; no sucking disk. The primary spines on the actinal side curved, with a large hoof. 

 Only tridentate and triphyllous pedicellarise are found. Of tridentate pedicellarise only one form is 

 found, with involuted edge, and the outer part widened in a spoon-like way, with straight and finely 

 serrate edge (PI. XIII. Figs. 8, 13). It is found of different sizes, up to i mm (the length of the head). 

 The neck rather long, the stalk as usual. (A figure of the whole pedicellaria is given by Koehler 

 (op. cit. PI. IX. Fig. 49)). The triphyllous pedicellariae with well developed cover-plate with many small 

 holes; the edge finely serrate (PI. XII. Fig. 42). The organs of Stewart well developed. It is evident 

 that this species cannot be referred to any of the preceding genera; it must form a new genus, for 

 which I propose the name of Hygrosoma 1 ), and its name will then be Hygrosoma Petersii (A. Ag.). 



<i,Phormosomav hoplacantha Wyv. Thomson seems to be very nearly allied to this species. Its 

 whole exterior is quite like it; the spines have a similar large, white hoof, and the primary spines 

 are arranged in the same way as in H. Petersii; also the tube feet are arranged quite as in the latter 

 species. Of pedicellarise only a large tridentate form is known, figured by Agassiz (Chall. Ech. 

 PI. XLIII. Fig. i, and PI. XLJV. Fig. 29). It seems to be very similar to the above described form in 

 H. Pctcrsii. Although I have not examined the pedicellarise of this species, I do not doubt that it 

 belongs to the same genus as Hygrosoma Petersii - the difficulty is rather to state any difference 

 between the two species. To judge by the figures of Agassiz, the pedicellarise , however, seem to 

 differ somewhat from those of H. Pctcrsii, so that presumably specific characters will be found in 

 these structures. As H. hoplacantha has only been taken in the Pacific (at Australia, Japan, and Juan 

 Fernandez), and as //. Pctcrsii is only known from the Atlantic, there can scarcely be any doubt that 

 they form two well distinguished species. 



No doubt iPkormosomat luculcntum A. Ag. is nearly allied to these two species. As in these 

 the spines of the actinal side end in a large, white hoof. The tube feet are arranged in the same 

 way; the spicules are rather large, irregular fenestrated plates, somewhat indistinctly arranged in two 

 series. A rather well developed sucking disk is found. The tridentate pedicellarise (PI. XIII. Fig. 14) 

 are very much similar to those of Hygrosoma Petersii; the triphyllous ones (PI. XII. Fig. 20) are of a 

 somewhat different form, but otherwise with large cover-plate and serrate edge as in H. Petersii. But 

 besides these forms still a very peculiar kind of pedicellarise is found (PI. XIII. Fig. 16), which is, no 

 doubt, a modified form of tridentate pedicellarise. The valves are very broad, constricted in the middle. 



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