68 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



side of each adambulacral plate there is a Unear brush of from 5 

 to 7 short, slender spines. 



Ambulacral grooves very narrow, tapering but very slowly. Am- 

 bulacral plates obscured by the rock but apparently very short, one 

 to each adambulacral and placed opposite one another. 



Genoholotype (type by monotypy) ami only species. — P. niagarensis 

 Hall. Siluric (Rochester shale). 



Remarlcs. — To this genus has been erroneously referred a multitude 

 of Paleozoic starfishes. It is to the starfishes what Terehratula and 

 RhyncJionella used to be among the brachiopods— a general dumping 

 ground for Paleozoic forms. This lack of generic conception regardmg 

 Palxaster lies somewhat in the fact that Hall's original diagnosis is 

 very meagre and that he did not describe nor illustrate the abactinal 

 surface, probably the most important area amongst Paleozoic aster- 

 ids for specific and generic differentiation. Of the twenty-four Amer- 

 ican species listed under Palseaster by S. A. Miller^ only one now 

 appears to belong there, the genotype. All others are here referred 

 to Hudsonaster, Urasterella, Meso palseaster, Promopalseaster, Anorth- 

 aster, Devonaster, and Neopalxaster. 



No new material of this genus accessible to the writer appears 

 to have been found smce Colonel Jewett discovered the origmal 

 two specimens of Palseaster niagarensis. The writer therefore had to 

 make the most of this material, and to determine the structure of 

 the abactinal area he w^orked away much of the blue shale from the 

 back of the one good spechnen. The finer detailed structure was then 

 revealed by cleanuig with caustic potash. 



In 1858 Billings proposed the genus Stenaster^ and writes that 

 "as it has been suggested that the two species hereuiafter described 

 should be referred to Palseaster, I give the f oUowing figure of the genus 

 in order to show the difference." This figure is a good reproduction 

 of Hall's figure 27. Billmgs then pomts out that "if the large plates 

 which border the grooves in Palseaster be adambulacral, then there 

 are only five oral plates, whereas m Stenaster there are ten. But if 

 they be not adambulacral but mai-gmal plates, then Palseaster must 

 have both mai-gmal and adambulacral [the correct view], while 

 Stenaster has only the latter." 



Hall's generic description is very meager and his figures give 

 the impression that the rays bear but a single column of mai-gmal 

 spiniferous plates. With this evidence one can understand why 

 Billmgs was m doubt as to whether these plates are inframargmals 

 or adambulacrals and therefore the uncertainty as to the nature of 

 the five large axiUary mai-ginal plates. If, however, he had read 

 Hall's accurate specific description, Billmgs would have seen that an 

 important discrepancy existed between the description and figures. 



> North American Geology and Palaeontology, 18S9, p. 265. 



2 Geol. Surv. Canada, Canadian Organic Remains, dec. 3, 1858, pp. 77, 78. 



