48 ECHINOIDEA. I. 



extending both in the ambulacral and interambulacral areas far towards the abactinal systems. This, 

 however, seems to be no conspicuous difference; on the other hand the ciirved spines on the abactinal 

 side and the peculiar feature with regard to the spicules may perhaps be taken to be good characters 

 of this species. 



One more species must be classed with the two mentioned ones, viz. Phormosoma rigidum A. 

 Agass. It resembles very much Ph. placenta. The primary spines on the actinal side are covered 

 with skin, what I have been able to substantiate on the type specimen 1 ). The pedicellariae as in Ph. 

 placenta; only a narrow form of tridentate pedicellarise has been found (PI. XII. Fig. 6). The spicules 

 are placed in three rather distinct longitudinal series; they are a little lengthened, and are almost 

 parallel to the longitudinal axis of the foot. No sucking disk. It seems to be rather difficult after 

 the only specimen in hand to give any sure character for the distinguishing between this species and 

 Ph. placenta, nor do we get any guidance from the description by Agassiz; to be sure he has 

 observed that the actinal side reminds very much of Ph. placenta, but otherwise he does not seem to 

 regard them as more nearly allied. In reality it is not improbable that they may be the same species. 

 Ph. rigidum, it is true, has only been taken at New-Zealand, and Ph. placenta only in the northern 

 part of the Atlantic, but if Ophiomusium Lymani can be found as well in the Atlantic as in the 

 Pacific (which is a sure fact) , the same may also be the case with Ph. placenta. New material , how- 

 ever, will be necessary for the decision of the question. 



The three mentioned species form a separate group, sharply distinguished from all other 

 Echinothurids, as far as known, above all by their peculiar, skin-covered spines on the actinal side. 

 Agassiz, to be sure, thinks that this feature has no special systematic importance. The presence of 

 sheated spines in two species of Phormosoma shows that this character, which at first sight seems to 

 separate so strikingly from the rest of the species of the group Asthenosoma grubei, is evidently one 

 of little value, and which may be more or less developed in specimens of the same species in the 

 same state of growth* (Chall. Ech. p. 101). --As already mentioned above, the facts here put together 

 by Agassiz are quite different: in A. grubei it is the spines on the abactinal side that are wrapped 

 by a bag of skin, and the spine itself is of the common structure, a perforated tube ending in a fine 

 point; in Ph. placenta and the species allied to it, it is the primary spines on the actinal side that are 

 clavately widened in the point and wrapped by a thick bag of skin. These spines must, of course, be 

 compared with the primary spines on the actinal side of the other species; but then we find a marked 

 contrast, these spines of the other species not being covered with skin --as far as is known - 

 but ending in a larger or smaller hoof, distinctly marked off from the spine itself. There can be no 

 doubt that the three mentioned species form a separate genus, to which, of course, the name of Phor- 

 mosoma is due. The other species referred to Phormosoma must be referred somewhere else. Possibly, 

 however, Ph. panamense is also a genuine Phormosoma; Agassiz (13) says that its actinal side has 

 the characters of Phormosoma most decidedly developed)); otherwise he takes it to be nearly related 

 to Ph. tenuc, but thinks that perhaps it may prove to belong to a new genns intermediate between 

 Phormosoma and Asthenosoma^. The description gives otherwise only very incomplete informations of 

 this species, and no figures are given. 



') As this specimen is said by Bell (69) to have disappeared, I must observe that it has later been found again. 



