20 PLATYCRIMD^. rLATYCRINITES. 



Formation and Localities. 



Carboniferous limestone. — Mendip Hills ; Hook Point, County of Wexford. 

 The remains of this species are less abundantly diffused than those of the preceding 

 one. 



Description of the Calcareous Frame Work. 



DoRso-cENTRAL PLATE. — The dorso-ceiitral plate of this species resembles that of the 

 P. Icevis, but the articulated depression for the attachment of the column is smaller. 



Perisomic plates. — These plates are thinner, and are slightly contracted towards 

 their upper edges, so as to impart a more decidedly rotound contour to the body of the 

 animal than has been observed in the typical species ; and the excavations and openings 

 communicating with the rays are neither so deep or so large. 



Meso-plates — or plates situated between the rays, and which Miller termed inter- 

 scapulars. In the present species these plates appear to be hexagonal with a central 

 blunt tubercle on each. The Meso-plates are invariably larger in the Platycrinites than 

 the plates lying between the rays and the base of the proboscis. They are, however, 

 smaller in this species than in the P. luvis. 



Abdominal plates — which cover the visceral cup between tlie base of the proboscis 

 and basal joints of the rays, have each a central thorn-like process slightly curved, and 

 frequently produced to a quarter of an inch in length. The exact office of these spines 

 in the animal economy it is difficult to conjecture, unless they were to protect its most 

 vvdnerable parts fi'om the assaidts of the lesser marine predatory animals. They could 

 not have been for the purpose of passmg the food to the mouth as they were immove- 

 able, and placed at too great a distance below the oral orifice to perform such an office. 



Some slight variations are occasionally observed in the length of the spines on the 

 abdominal plates. 



Proboscis, or Oral Tube. — This organ furnishes very conspicuous characters in the 

 present species. It is central and much elongated, the plates covering the upper portion 

 are each armed with a long central immoveable spine, somewhat resembling in its slight 

 curve the spur of a fowl. The plates surrounding the middle portion are smooth and 

 hexagonal, gradually becoming, as they approach the base, tubercidated in their centres. 



