54 KCHINOIDEA. I. 



coalesced, so that a series of large holes is found along the median line -- and they may also be 

 found of the form, figured of A. coriaceum). The pedicellarise (the tridentate ones) with short neck; 

 the stalk of the common structure. The tube feet in three series. The spicules (PI. XI. Fig. 15) are 

 not so compact fenestrated plates as in A. fenestratum, the holes are much larger and fewer. In the 

 lower part of the tube foot the spicules are more narrow, at last only fine, thorny, irregular needles, 

 often a little widened as small fenestrated plates in one end or in both ends, or the}- have a larger hole 

 in the middle. Below they seem to be arranged in four longitudinal series, above they inclose the 

 whole foot as a close mail. The sucking disk well developed in the actinal tube feet. The primary 

 spiues on the actinal side form a very conspicuous, regular series along the outer edge of the interambu- 

 lacral areas; in the ambulacral areas only 5 — 6 large spines are found scattered on the outer plates. 

 They are curved, and end in a little hoof. Resembling more nearly the primary spines of Phormosoma 

 than the characteristic flaring trumpet-shaped spines of Asthenosoma », Agassiz says of these spines 

 (Chall. Ech. p. 88). As his <> Phormosoma* contains so widely different forms as Ph. placenta and hopla- 

 cantha this statement gives no clear information; the meaning of it is that the) - are similar to those 

 of A. fenestratum ; the hoof is little, short, and broad. 



Agassiz says of this species that it is «allied to Asthenosoma gntbii in having an extremely 

 thick leathery cuticle^ (1. c); according to the informations given here there is no nearer relation 



between these two species. Agassiz further thinks that it is quite possible that this may be the 



adult of Asthenosoma tesselatum ■■> (1. a). After having examined the type specimen of this species I 

 can say with certainty that this is not the case; the two species are not even so very nearly related 

 even if they possibly belong to the same genus. - Tetradactylous pedicellaria; have not been found 

 in this species. The tridentate pedieellariae occur in two forms, between which there seem to be no 

 transitions. The large form is quite similar to the large tridentate pedicellariae in A. fenestratum 

 (PI. XIII. Fig. 5); the smaller form (PI. XIII. Fig. 6, PI. XIV. Fig. 15) is very peculiar, the blade deep, 

 filled by a rich net of meshes, and with a highly irregular edge without such large sinuations as are 

 found in A. fenestratum and coriaceurn; the wideniugs from the upper end of the apophysis continue 

 directly into the edge of the blade. When the pedicellaria is shut, the edges join completely, there is 

 only at the basal part a small open space. This form is a little more long-necked than usual. The 

 triphyllous pedicellarise are quite similar to those of A. fenestratum and coriaceurn; the stalk of the 

 pedicellaria; of the common structure. Spicules and sucking disk as in A. fenestratum; the tube feet 

 in three series. All the primary spines on the actinal side are broken in the only specimen known, 

 so that it is impossible to say anything of the form of the point; surely, however, they are not skin- 

 covered. — For the present it is impossible to decide whether this species is to be classed with A. 

 fenestratum and coriaceurn; but several things speak in favour of this supposition, and it will therefore 

 be most correct provisionally to refer this species to the genus Arceosoma. That the membranous 

 interspaces between the plates are especially large in this species speaks, of course, only in favour of 

 the supposition that it really belongs to this genus. 



Among the specimens kept in British Museum under the name of Asthenosoma hystr/.v, a piece- 

 was found (from Barbados, 137 fathoms), which is no doubt a new species, and probably also belongs 

 to this genus. It is very similar to Calveria hvstr/x, but is of a darker colour (brownish violet). 



