CONTRIBUTIONS TO PALEONTOLOGY. 325 



In Decade iii, Canadian Fossils, Mr. Billings has reproduced the figure 

 of Pal^aster, Hall, to show the differences between it and his Genus 

 Stenaster ; and the wood-cut is so treated as to obscure the indications 

 of adambulacral plates shown in the original figure. 



In Stenaster = Urasterella, however, we have the absence of adam- 

 bulacral plates, which should be regarded as of generic importance. 



In the Genus Petraster of Mr. Billings, we have " both marginal 

 and adambulacral plates" [as in Palseaster], " with a few disc-plates on 

 the ventral side." * * * * " It differs from Palasterina 

 by the presence of large marginal plates outside of the disc-plates ; and 

 still more from Stenaster, which has neither disc nor marginal plates." 



The character of Petraster here described, and illustrated in the figure 

 (3 a, Plate 9), gives an intercalated partial range of disc-plates, between 

 the adambulacral and marginal plates, which will separate these forms 

 from any of the Pal^asters in my collection. Figure 3 ^ of the same 

 plate (Decade iii) shows no intercalated plates ; and though supposed by 

 Mr. Billings to be the dorsal side of the same " species, with the plates 

 along the centre of the rays removed," it has much the appearance of 

 the ventral side of a species of Pal^aster, and has a different aspect 

 from fig. 3 a. 



The Genus Pal^aster has two ranges of plates on each side of the 

 ambulacral groove ; marginal and adambulacral plates on the lower side 

 besides ambulacral or poral plates. The upper or dorsal side has three 

 or more ranges of plates. 



In several species examined, the ventral side presents no disc-plates 

 or any plates in the axils of the rays which do not belong to the marginal 

 series, or to the plates of the dorsal side, which are sometimes pressed 

 beyond the marginal plates of the lower side. 



The following species belong to this genus : 



Pal^aster matdtinus. 



PLATE IX, FIG. 2. 

 jisterias matutina, Hall. Palaeontology of New York, I, p. 91, Plate 29, fig. 5. 



The specimen described shows the dorsal side only ; the rays are regu- 

 larly tapering, rather slender, having three ranges of plates with two 

 stronger plates at each axil. The body or disc above consists of strong 

 plates. The marginal plates of the ventral side are visible on the side 

 of the rays below the outer range of dorsal plates. 



