INVERTEBRATES. 357 



Heteroschisma alternatum and H. elonc/atum, but the summit of the 

 pyramids extends considerably above the crests of the oral plates, 

 while in the two others the summit scarcely reaches the level of 

 the peristome ; it also has narrower, almost linear ambulacra, 

 against slightly petaloid ones in the others, and these are divided- 

 by the food groove only, while the Louisville species have three in- 

 dented lines along their surface. It further differs from H. alter- 

 natum in having straight, and not convex sides along the calyx, 

 and a conical, more sharply pointed basal cup. 



It is barely possible that Pentremites subtruncatus, Hall (Geol. Rep. 

 Iowa, Vol. I, Pt. II, p. 435), is identical with this species. A com- 

 parison is impossible as Hall's description is so indistinct, that not 

 even the genus can be identified. He refers it to forms like Pen- 

 tremites Rheinwardti, and mentions "ovarian openings," which if 

 present suggest a very different thing. Should, however, hereafter, 

 the identity be proved, the name must be placed as a synonym under 

 Heteroschisma gracile. 



Position and locality, etc. : Found near the top of the Hamilton 

 group, Alpena, Mich. 



(The type specimens are in the collections of Prof. Barris, and 

 Charles Wachsmuth.) 



Description of some new Blastoids from tho Hamilton Group, by W. H. BARKIS. 



EL^ACRINUS, Roemer.* 



SYN. Pentremites Troost, 1841, Cth Rep. Geol. Tennessee, 



Nucleocrinus Conrad, 1843, Jour. Acad. Nat. Sci. Phila. Vol. VIII, p. 280. 



Olivanites Troost, 1850, Cat. name. 



Elaoacrinus Roemer, 1851, Monog. Blastoidea, p. 55. 



Olivanites Lyon, 1857. Geol. Sur. Kent, Vol. Ill, p. 490. 



Nucleocrinus Lyon and Cass, 1859; Hall, 1862; Shumard, 1862. 



EliBacrinus Shumard, 1866, Trans. St. L. Acad. Sci., p. 368. 



Nucleocrinus, Canadensis? Montgomery, 1881; Can. Nat. Vol. X, No. 2. 



*At the suggestion of Mr. Charles Wachsmuth, and in conformity with the views of the 

 late Dr. Shumard (Cat. Palaeoz. Fuss., in the Trans. St. L. Acad. Sci., 1866, p. 368) . I 

 give Roemer's later name Elwacrinus preference over Nucleocnnus conraci. uonraas 

 entire description is as follows: "Nucleocrinus Conrad. The genus differs from Fenti - 

 mites. Say, in having only one perforation, which is central. In this description the only 

 distinguishing character is erroneous, the accompanying figure poor and 'ttOOrreot,Mi(l 

 Hall's excellent descriptions of Nucleocrinus were published ten years later than tn 

 Elaeacrinus. Roemer, in proposing the latter name, gave a ^ood definition of the genus 

 He found the central aperture which Conrad had pronounced the only Perfora 

 the top," closed in perfect specimens, and surrounding it he discovered one large ana t 

 smaller openings. 



