CONTRIBUTION'S TO PALEONTOLOGY. 337 



ambulacral and adambulacral, on each side of the ambulacral groove, 

 which are opposite each other, with a large oval pore between them, while 

 the outer margins of the adambulacral plates are garnished with spines. 

 Notwithstanding these differences, which would be of generic import- 

 ance, I am still inclined to refer our species to the Genus Protaster, 

 knowing how dificult it is for the palaBontologist, with imperfect 

 material at his disposal, to give always the true interpretation and repre- 

 sentation of parts, which further examination and additional material 

 enable him to furnish. I am disposed to believe that the plates repre- 

 sented as opposite will be found slightly alternating, and that the 

 structure of the lower side of the ray in Protaster miltoni will prove to 

 be not very dissimilar from that of Protaster forhesi. 



NOTE ON THE GENUS PETRASTEE, Billings, 



In examining the several species of Pal^aster, certain features were 

 revealed which showed a very intimate relation with the species described 

 by Mr. Billings under the generic name of Petraster. Through the 

 kindness of Sir William E. Logan, I have been permitted to examine 

 the originals of Petraster rigidus (figs. 3 a and 3 h^ Plate ix. Decade iii, 

 Canadian Organic Remains). 



The specimen, fig. 3 h, is the ventral side of a true Pal^aster, having all 

 the essential features of the genus, and none other. It is a small individual 

 of Palceaster matutina, presenting all the characteristics of that species. 



The specimen illustrated in fig. 3 a, has a few small intercalated plates 

 between the marginal and ambulacral ranges in two of the axils of the 

 rays; and there are a smaller number of granules in a similar position, 

 but unequally distributed on one side of another axil ; while two of the 

 axils do not show any such intercalated plates or granules. In one of 

 the rays, at least, the ambulacral, adambulacral and marginal plates are 

 distinctly visible, without intercalated plates on either side. 



The presence of these unequally distributed plates or granules is 

 apparently an abnormal structure, probably the result of accident during 

 the growth of the animal ; and this view is sustained by the fact that the 

 other parts have the ordinary structure of Palceaster, and in all other 

 respects the specimen agrees with the typical Palceaster matutina. 



Cab. Nat. 43 



