294 TWENTIETH REPORT ON THE STATE CABINET. 



species, Mr. Salter says the arms '* are made up of a double row of about 

 *' forty pairs of squarish concave plates above, placed exactly opposite, not 

 " alternating as in other species " ( PL ix, fig. 4 Z> ). On the lower side, the 

 ray is represented as made up of two ranges of plates, ambulacral and 

 adambulacral, on each side 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 woul(i be of generic importance, 

 I am still inclined to refer our species to the Genus Protaster ; knowing 

 how difficult it is for the palaeontologist, with the imperfect material at his 

 disposal, to give always the true interpretation and representation of parts, 

 which farther examination and additional material enable him to furnish. 

 I am disposed to believe that the plates represented 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 

 forbesi . 



Protaster Forbesi ; Hall. Plate ix, figs. 5 and 6. 



NOTE ON THE GENUS PETRASTER (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 ( figures 3 a and 3 ^, plate ix, Decade iii, 

 Canadian Organic Bemains). 



The specimen, fig. 3 ^, 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 tiot show any such intercalated plates or granules. In one of the 

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

 tinctly visible, without intercalated plates on either side. 



The presence of these unequally distributed plates, or granules, is appa- 

 rently 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 Pal^aster, and in all other 

 respects the specimen agrees with the typical Palceaster matutina, 



*Annals and Magazine of Nat. History^ second series, pa. 330, pi. ix, f. 4 c. 

 14 



