PIATYCRINID.E. — HEXACRINl'S. 51 



4. Species. Hexacrinus ? pentangularis. (Austin.) 

 PL Vn,fg. \,a,h,c,i. 



Definition. — Dorso-central plate tripartite?; perisomic plates six, five of which bear 

 the rays. These latter plates are prominent near the excavations for the attachment of 

 the rays, which imparts to the calcareous skeleton a decided pentagonal contour. All 

 these plates are smooth and thin. 



Synonymes and References. 

 Platycrinus pentangularis.— Phill. Pal. Foss. PL 60,/. 29*/. 39**, p. 240. 



Formation and Localities. 

 Devonian System. — Newton Dushell, Devon. 



The specimen from which our figures are taken does not shew the divisions in the 

 dorso-central plate, which is broken and otherwise imperfect, nor does it exhibit the 

 arrangement and divisions of the perisomic plates as distinctly as we could wish ; but 

 notwithstanding vve consider the evidence is in favor of its belonging to the genus 

 Hexacrinus, and against its being a Platycrinus. 



The excavations for the rays are shallow; but shallow as they are, the lower ray joints 

 do not fill up the lunulate depressions. On each thin lower articulation is a cuneiform 

 joint, which is also exceedingly thin, with a central perforation. From this point, the 

 rays bifurcate, the succeeding joints being oval, and articulating bj^ marginal stria?. 

 These joints are perforated to correspond with the passage through the cuneiform joints 

 below them, and they also appear to be sulcated on their inner sides. 



When enumerating the genera comprised in the family Platycrinid<e, the genus 

 Hexacrinus was omitted, as it was not then established. This was owing to our 

 erroneously supposing the different species since removed to the new genus, were 

 properly retained among the Platycrini. 



v^jnong the fossils from the Devonian rocks in Mr. Austen's collection, are two 

 apparently undivided hexagonal dorso-central plates. These specimens have been acted 

 on by some phj'sical agent, probably infiltered water, which has in part decomposed 

 and re-arranged the particles in such a manner as to obliterate all trace of the di\isious. 



