PALEOZOIC FOSSILS. 283 



In Decade iii, Canadian Fossils, Mr. Billings has reproduced the figur-3 

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

 Stenaster ; and the woodcut 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 Palgeaster], "with a few disc-plates on 

 *' the ventral side." * * * * *• It differs from Pala- 



*' STERINA 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, pi. 9), gives an intercalated partial range of disc-plates, between the adam- 

 bulacral and marginal plates, which will separate these forms from any of 

 the Palaeasters in my collection. Figure 3Z> 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 Palgeaster has two ranges of plates on each side of the ambu- 

 lacral 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 th» 

 marginal plates of the lower side. 



TheJoUowing species belong to this genus : 



PAL.ffi)ASTER MATUTINA. 



PLATE IX, FIG. 2. 



dsterias matutina : Hall, Palgeontology of New-York, Vol. i, pa. 91, pi. 29, f. 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. 



The ventral surface shows a range of marginal and one of adambulacral 

 plates, the latter much smaller, but the same in number and alternating 

 with them. Two ranges of poral plates are visible in the groove, each one 

 being more deeply excavated on the inner or posterior, than on the outer 

 margin, to form the pore. 



