PL.VTYCUINID^,. PLATYCKINITES. 



Synonymes and References. 



23 



Platycrinites Irevis.— Phillips, Geol. Yorkshire, PL Ill.Jiif. 14, 15. page 204. 



mucronatus. — Messrs. Austin, Ann. ^ Mag. Nat. Hist. No. 6S, page 109, 



and No. 69, j9. 199. Vol. 10 Sf 11. 



Formation and Localities. 



Carboniferous limestone. — Bolland ; and Mendip Hills \ 



This species appears to have been of rare occurrence, and extremely local as regards 

 station. It has hitherto only been observed in the localities before named, and very few 

 specimens have been obtained even there. We have traversed the denuded strata of 

 moimtam limestone, so frequently met with in Ireland, for many miles without finding 

 the least indication of the species. 



Description of the Calcareous Frame Work. 



DoRso-CENTRAL PLATE — pentagonal and saucer shaped, the depression for the attach- 

 ment of the column large, but the perforation communicating with the columnar canal 

 is smaU and ciicidar. The radiating stria? for the articidation of the column neat, but 

 well defined. 



Perisomic PLATES. — These plates more closely approach to the hexagonal form, than 

 to the nearly quadrilateral shape of the perisomic plates in the P. Icevis. In the 

 mucronatus they are extremely short, smooth, and deeply excavated on theii- upper edges 

 for the attachment of the basal joints of the rays. The apertures of communication 

 between the intestinal cavity and the rays are sufficiently large to have admitted pow- 

 erfid muscles for imparting motion to the extremities. 



The Meso-plates — interscapulars of Miller, are five in number, and large as compared 

 with those of oth(?r species, for no Plafi/crinus of equal size, has yet been observed to 

 have them at all in proportion to the mucronatus. These plates occupy the spaces 

 between the rays, and from their size were evidently intended to impart strength and 

 stability to the base of the rays. The upper ends of those septagonal plates are trun- 

 cated, on which the abdominal plates rest. Thek lower ends are pointed, by which 

 they fit into the retiimg angles formed by the imion of the perisomic plates. 



