1881.] NATURAL SCIENCES OF PHILADELPHIA. 373 



Interradial arese composed of but few plates, the first one large, 

 placed between the second radial s, and leaning partl}^ against the 

 third, with two small plates above. Posterior or anal area wider, 

 the first plate in line with the first radials and of the same size ; 

 it is followed by three plates in the second series and several 

 smaller ones above, the upper ones connecting with the vault. 

 Vault, and form of the anus unknown. 



Geological Position^ etc. — The only three known species are 

 Upper Silurian, from the age of the Niagara group, 



1839. Dimerocrinus decadactylus Phillips. Type of the genu?. Murehison's 



Silur. S^-st., p. (574, PI. 17, fig. 4; d Orbigny, 1850, Prodr. de Paleunt.. i, p. 



46; Piete', Traits dd Paleont., i, p. 368. Upper Silurian. Dudley, Eng. 

 *1852. Dimerocr. liliiformis Hall. (Thysanoor. liliiformis) Palemt. N. Y., ii, p. 



188, PI. 42, figs. 1 a-f. Niagara gr. Lockport, New York. 

 *1852. Dimerocr. immaturus Hall. (Thysanoor. immaturus) Pa'eont. N. Y., ii, 



p. 191, I'l. 42, figs. 4 a-f. Niagara gr. Lockport, N. Y. 



6. LAMPTEKOCEINUS Roemer. 



1860. Roemer. Silur. Fauna West. Tenn., p. 37. 



1863. Hall. Trans. Albany lust., iv, p. 202. 



1868. Hall. 20th Rep., N. Y. St. Cab. Nat. Hist., p. 328. 



Syn. Balanocrinus Troost, 1850 (not Agassiz, 1846), Cat. Foss. 

 Tenn. 



Hall, in comparing the genera Glyptocrinus^ Glyptader^ Balan- 

 ocrinus and Lampterocrinus (Trans. Albany Inst., iv, p. 202), 

 considered the last two identical, and that the}' differ from the 

 former in having larger underbasals. He then discriminated in 

 favor of Troost's Catalogue name, but afterwards in the 20th 

 Rep. N. Y. St. Cab., p. 820, he very properly, recognized the 

 priority- of Roemer's name Lampterocrin us. A difference as above 

 stated would scarcely be sufficient to warrant a generic separation, 

 but we think this genus is separable from the other two by other 

 and more important characters than those noted by Hall. Lamp- 

 terocrinus differs from Glyptocrinus in the arrangement of its 

 anal plates, and in having an anal tube ; from Glyptaster in the 

 anal and arm structure, and from both of them in the size of the 

 basals and underbasals, in the form and construction of the vault, 

 and in the pentagonal column. 



Roemer describes Lampterocrinus as having four primary 

 radials, the fourth bifurcating and excavated for the reception of 

 an arm. This is not quite in accordance with our views, we 



