NIAOABA OXOUP. 



247 



ASTERIAD^. 



FOSSIL STARFISHES. 



We now know that species of this family have existed in every geological period, from the 

 Lower Silurian upwards to the Tertiary period, as well as in the present seas. Prof. Forbes has 

 published (in the Memoirs of the Geological Survey of Great Britain) descriptions of all the 

 known British Fossil Asteriadffi, which amount to the number of thirty-five species. He has 

 referred the palaeozoic species to the existing genus Uraster, regarding them as differing in 

 no essential feature from the living forms. It might appear unnecessary, therefore, to propose 

 another name for our palaeozoic species. Prof. Agassiz, however, holds a different opinion ; 

 and, after a careful examination of a very fine specimen from the Hamilton group, informed 

 me that he had discovered an important characteristic, by which not only that one, but other 

 palffiozoic species could be distinguished from recent forms. This feature consists principally 

 in the plates being perforated by pores ; while in existing genera, the pores penetrate the 

 joints, or pass between the plates. He also regards the several fossil species known to us here 

 as constituting a genus quite distuict from any established one. Contenting myself with this 

 authority, I shall, with the consent of Prof. A., propose the name of Pal.easter for our Silu- 

 rian species ; since his investigations have not yet been put in form for publication. 



Genus PALjEASTER (nov. g-en.). 



Body stellate ; rays five, rounded, spinous, composed of five or more series of plates ; avenues 

 deep, and bordered by strong spines ; pores penetrating the plates of the upper surface. 



609. 1. PALiEASTER NIAGARENSIS (n. 5;>.). 



PL. LI. Figs. 21 -23. 



Body stellate ; disc small ; arms short, terete with a deep avenue on the lower side, which is 

 margined by strong short spines ; centre of plates (in the fossil) nearly smooth, margins strongly 

 granulate ; lower side of arms showing two ranges of plates on each side of the avenue, the 

 outer range composed of strong hexagonal plates, with an inner range of smaller ones alter- 

 nating, the latter usually covered by tufts of spines ; a large pentagonal plate inserted at the 

 base of the arms, on the lower side. 



In this species, the arms are about twice as long as the width of the disc. There are about 

 fifteen or sixteen plates in each range from the base to the apex of the arms. The range of 

 plates margining the avenue are usually not visible, the whole being covered by the short 

 spines, which also partially fill the avenue. A single specimen only has been discovered ; but 

 it is quite distinct from the species in the Lower Silurian strata, or from those known in a 

 higher position. 



