Mr. J. W. Salter on some neiv Palceozoic Star-fishes. 323 



analogy in the northern form Pteraster, which Miiller has sepa- 

 rated from the other species of Asteriscus. 



Prutaster, in which the arms stretch far beyond the plated 

 disk, presents some points of analogy with the latter-named 

 genus, but it is clearly referable to the Ophiurida, and must be 

 regarded as a new form of that family, in which the lower or 

 basal plate (as well as the upper one) has become longitudinally 

 divided f. The ambulacral pores (or ])assages for the feet) do 

 not lie between the rows of plates, as in the Asteriada, but on 

 the outside of them (PI. IX. fig. 7), as in Ophiura (fig. 8). 



Exclusive of the form last quoted, the Silurian Star-fishes 

 present one character which effectually distinguishes them from 

 Asteriscus, and indeed from any other group of Asteriadcc, un- 

 less it be the Pteraster above mentioned ; it is the possession 

 of the row of large adambulacral J plates, or plates bordering 

 the avenues before alluded to {Goniaster and its allies have the 

 large plates marginal). If we compare, for instance, Palaaster 

 with the living Asteriscus {Patiria) crassus, there is a near agree- 

 ment in outline, and in the rough upper surface covered with 

 close-set rows of prominent tubercles. But neither in this spe- 

 cies nor in A. graniferus, or any other species I have seen, are 

 the inner or adambulacral rows enlarged, although the basal 

 pairs of each row (fig. 3, c*) are increased in size, and bear combs 

 of spines (in A. Gunnii for instance, which in this point exactly 

 resembles our fossil). 



This striking character should probably indicate a subfamily, 

 which might be called Palusterince, and may hereafter be again 

 subdivided according to the development or otherwise of the 

 arms or disk respectively. 



Fam. AsTERiAD^. 

 Genus 1. Palaaster ^, Hall. Arms thick, convex, short or 



t- The number of bones in the complete circlet for the arm in this genus 

 (fig. 7) is at least six ; to account for which, if we regard them as Ophiura, 

 we must suppose the upper or dorsal plate to be divided, as well as the 

 lower. Instances of accidental fission of the upper plate are indeed some- 

 times met with in recent Ophiurce — Ophiolepis for instance ; but no species 

 is known which has it normally divided. The analogy with the Euryales 

 is very remote, nor do I quite see in what way Forbes considered them 

 allied, since no Euryales have the arms covered by plates externally. 



X Forbes does not seem to have recognized this character, but to have 

 mistaken these large adambulacrals for the ossicles of the avenues them- 

 selves : " Ossiculis ambulacrahbus oblongis, latis, interstitiis linearibus," 

 Decade i. pi. 1. p. 2. The true ambulacral ossicles are deep-set, and much 

 smaller. 



§ Prof. Hall's description of an Upper Silurian Star-fish under this name 



21* 



