66 PALEONTOLOGY OF THE UPPER MISSOURI. 



REPTILIAN AGE 



(JURASSIC PERIOD.) 



RADIATA. 



Class ECHI]\0DERIIIATA. 



Order Crinoiclea. 



Family PENTACRINIDiE, 

 Genus PENTACRINITES, Miller, 



Si/non. — Pentacrinites (Schlot.), Miller, Nat. Hist. Crinoid. 1821, 56. — Goldf. Petref. Germ. 1, 1826, 168. — Roemer, 

 Ool. 1836, 29 ; Kreid. 1841, 2S.— Bron. Leth. Geog. 1836, 219. 



Pentacrinus, Agassiz, Prodr. Mong. Ech. Mem. Soc. Sci. Nat. Neuchatel, 1835, 195 ; and various later authors. 



C'hladocrinus, Ao. lb. 196. 

 Eti/m. — irivTic, five ; xftm, a Lily. 

 £xamp. — Pentacrinites briareus, Milleb. 



Column more or less distinctly pentagonal, with central cavity small and 

 roimded ; provided with lateral branches or accessory arms arranged in verticils ; 

 segments ornamented with star-Hke sculpturing on their upper and lower surfaces. 

 Body small, composed of five small or rudimentary basal plates, and fifteen larger 

 radials, in five series of three each, without inter-radial pieces. Visceral cavity 

 protected by a covering of numerous very small polygonal plates. Arms large, 

 long, frequently bifurcating, and provided with numerous jointed tentacles. 



Prof. Agassiz separates this group into two sections, as foUows : — 



1. Pentacrinites (proper). 



Column with lateral branches simple. 



2. Cbladocriniis, or Cladocriniis. 



Lateral branches of column themselves provided with verticillate branohlets. 



Probably the most ancient known species of this genus are from the St. Cassian 

 beds of the Tyrol, often referred to the Trias, but by some included in the 

 Jurassic system. The genus attained its greatest development during the deposition 

 of the Jurassic rocks ; but occurs in the Cretaceous and Tertiary deposits, and is 

 represented by a few species in our existing seas. It has not been found in this 

 country east of the Black Hills.^ 



' It is an error, we think, to quote Pentagonites, Rafinesque (Jour, de Phys. LXXXVIII, 1819, 

 429), described by him, with other fossils from some of the Palteozoic rocks of the Western States, as 

 a synonym of Pentacrinites. On the contrary, his type was more probably one of the Silurian Crinoids 



