PLATYCRINID.E, — CYATIIOCRINUS. 59 



Synonymes and References. 



Cyathocrinus planus.— Mill. Cr'm. p. 86, fg. 29, 30. 

 Bronn. Let. Geogn. t. A,f. G. 

 Cyathocrinus distoitus. — Phil). Geol. York. 2, t. 3, /. 34. 



Formation and Localities. 



Carboniferous or mountain limestone. — Black Rock, banks of the Avon, Bristol ; 

 Barry Island, coast of South Wales; Castleton, Derby; Hook Point, Wexford; 

 Lilleshall, Salop ? 



Magnesian beds of the mountain limestone, Clevedon Bay ; and Woodspring. 



Magnesian limestone.^ — Humbleton ? 



There has been much confusion respecting the localities where the C. planus occurs ; 

 this is owing to several very different crinoids having been referred to this species. We 

 believe that none of the genus occur in formations newer than the carboniferous lime- 

 stone, or of greater age than the Devonian. 



Description of the Calcareous Skeleton. 



The Dorso-central Plate of the C. planus is quinque-partite and shallow, witli 

 a circular ? perforation in the centre, communicating with the columnar canal. — 

 Exteriorly and centrally, is a slight dejiression, striated for the reception of the superior 

 columnar joint. 



The Perisomic Plates. — The first series consists of five, four of which are pen- 

 tagonal, but inclining to a subhexagonal form ; the fifth })latc is hexagonal, though 

 sometimes rounded at its lower edge. 



The whole first series of plates are prominent in their centres, which imparts an 

 appearance of fulness to the body. 



The second series consist of six plates, five of which bear the rays. The non-ray- 

 bearing plate is hexagonal, and rests on the truncated upper edge of the hexagonal plate 

 of the first series. 



The Ray-be arixg Plates are perforated low down from their upper edges, for the 

 passage of the muscles of the rays. A deep groove runs from each perforation to the 

 summit of each plate, which is prolonged considerably beyond the point of attachment 



