ECHINOIDEA. I. 47 



of Phormosoma placenta I have opened, had typicall}- developed, but, to be sure, very fine and fragile 

 longitudinal muscles. Bell (69) has likewise shown tliat the organs of Stewart are rudimentary or 

 wanting in Ph. placenta^ bursariuiii , and ioiuc^ while in other forms they are highly developed, as 

 has been shown by S ar as in (352) with regard to A. urens ^ and by Koehler witli regard to «/V/. 

 jiramis') {229). 



If we now look over the Echinothurids with regard to the structures mentioned here, we shall 

 see that the old genera Plioriiiosoina and Asf/icnosoiua cannot be kept up to the extent in which they 

 have hitherto been taken; several new genera will have to be established. The species will have to be 

 grouped in the following \va.\: 



Phormosoma placenta. The primary spines on the actinal side are club-shaped, inclosed by a 

 thick bag of skin. The tube feet on the actinal side arranged in a single series; no sucking disk 

 developed. Tridentate pedicellariæ simple, with leaf-shaped, rather deep valves having only a slightly 

 developed net of meshes at the bottom. The spreadings from the upper end of the apophysis do not 

 reach to the lateral edges of the blade. \'er\- nearh' allied to this species is Ph. hursarmm A. Ag. 

 The spines on the actinal side are as in PJi. placenta; on the abactinal side the spines are curved, b\- 

 which feature it is distinguished from the latter species. The pedicellariæ are as in PJi. placenta \ the 

 tridentate pedicellariæ occur (in the same individual) in a long, narrow form (PI. XII, Fig. i), and a 

 short, broad form, as it will be de.scribed below in Ph. placenta (PI. XII. Figs. 2, 3); (in this species 

 both forms do not appear to be found in the same individnals). The narrow ones have often some 

 rather large, inward directed teeth a little inside of the edge on the lower part of the blade. In the 

 triphyllous pedicellariæ (PI. XII, Fig. 28) the cover-plate is a little more developed than is the case in 

 the form typical for Ph. placenta; but in this species similar triphyllous pedicellariæ may also be found 

 together with the typical form. The spicules form two longitudinal series placed just above either 

 edge of the partition-wall in the tube foot; from the middle of the lower side of the spicules a continu- 

 ation passes into the partitiou-wall, by which means a dark line appears along the middle of each 

 series of spicules. Such continuations from the spicules into the partition-wall are not seen in Ph. 

 placenta, and seem upon the whole not to be found in other of the Echinothurids examined here. 

 Bcsides the two longitudinal series more or fewer scattered spicules are found, sometimes so many, 

 that the chief series become indistinct. The spicules are the common irregular fenestrated piates, 

 perhaps a little larger than in Ph. placenta. No sucking disk is found. 



Agassiz (Chall. Ech. p. 99 seq.) is not quite sure whether this species is not possibly identical 

 with Phormosoma biculentum; «more abundant material may prove, that the differences noticed, 

 although important, are simply individual characteristics partly due to age . He takes much care to 

 show, in which features the two species are distinguished — a rather superfluous work! The two 

 species are very different, which may be seen directly by a glance at the figures given by Agassiz, 

 and, as will be shown here, they cannot even be referred to the same genus. On the other hånd 

 Agassiz unfortunately has not observed that Ph. hursarmm is very similar to placenta; it would have 

 been of considerably higher importance, if we had been informed of the characters by which it is 

 distinguished from this latter. To be sure Agassiz (Chall. Ech. p. 100) observes that it is distinguished 

 from Ph. placenta (in the greater height of its coronal piates and the presence of large primary tubercles 



