Mr. J. W. Salter on some new Palaozok Star-fishes. 485 



which it is normally so. The six plates in a circuit were there- 

 fore supposed to result froni the division of the upper and lower 

 plates each into two pieces. But there was a manifest resem- 

 blance in the lower plates of the arm to the corresponding 

 ambulacral plates or ossicles of Palaocoma {ib. fig. 3), and even 

 Palasterina (fig. 2). Again, the passages for the ambulacral feet 

 were supposed to be outside these two plates, between them and 

 the marginal plates, which would be the right position for 

 Ophiura, or a modified form of Ophiurida, as this was supposed 

 to be. 



Our fresh specimens clear up this point likewise, and show 

 that in the structure of the ambulacra, as well as in the form, 

 Pi'otaster was only imitative of the Ophiunda. 



The real shape of the ambulacral bones (fig. 11) is given from 

 a perfect specimen ; and a comparison between figs. 10 and 11 will 

 show how the passages may appear to be outside the ossicles, and 

 yet be really between them, as usual in Asteriadce. The great 

 size of the apertures encroaches so much on the length and 

 breadth of the ossicle as to excavate it in the manner shown in 

 fig. llj which is a magnified view of two pairs of these bones. 

 There is manifestly no room for the feet to protrude at b ; and 

 hence, till the narrow overhanging piece (c) was shown by these 

 specimens, the aperture appeared to be outside the plate, as in 

 Ophiura. 



It is curious enough, but should hardly be surprising, to find 

 a form belonging to one family so closely simulating those of 

 another, even to minute details. No one, I am persuaded, look- 

 ing merely to the general shape of these long-armed species, 

 with their round disks apparently covered with scales *, and the 

 twisting arms fringed with stifi" spines, but would have referred 

 them to the Ophiurid group. If he looked closer, he would find 

 the plates composing the arms flattened squamre, rather than 

 thick ossicles, bearing combs of spines exactly like those borne 

 by Oj)hiurid(E. The oral apparatus, if not quite like that of an 

 Ophiurid, is at least very unlike that of a Star-fish ; and there 

 are even the pencils of spines which are conspicuous on the oral 

 ossicles of the former group. Every character of the Asteriad 

 group has been distorted, so to speak, in order to simulate that 

 of another group ; and when we detect Nature's innocent fraud, 

 we can but wonder the more at her ingenuity. 



I may just mention, in passing, the presence of the madrepori- 

 form tubercle on those curious forms figured lately by Prof. 



* They are not really so, though Prof. E. Forbes described and figured 

 them as such in Decade I. of the Geological Survey. The skeleton even 

 of the disk is closely reticular, and the disk is membranous between the 

 bones. 



