28 PLATYCRINID^. PLATYCRINITES. 



Formation and Localities. 



Carboiiiferous limestone, Mendip Hills ; Hook Point, Co. AVexford 1 and Yorkshire. 



This species appears to have been of extremely local occurrence, as the only specimens 

 hitherto obtained are from the Mendips and Yorkshire. We have an imperfect speci- 

 men from Ireland, which is probably identical with this species ; but the characters are 

 not sufficiently clear to enable us to determine with certainty as to its affinity. With 

 this sino-le exception, we have met ^vith no indication of the species throughout the 

 mountain limestone of Ireland. 



Description of the Calcareous Frame Work. 



The Dorso-central plate. — In this species the dorso-central plate is proportionably 

 smaller than in the mucronatus. The depression for the adlicsion of the column is small, 

 and the articidating strise finely marked. The opening into the columnar canal is also 

 small. 



The Perisomic plates — are short and spreading towards their upper edges. These 

 plates are nearly hexagonal, as they are in almost all the known species with mouths 

 but sho-htly produced. This pecuharity is strikingly CAddent when they are compared 

 with the nearly quadi'ilateral perisomic plates of the typical species. Though the num- 

 ber of plates below the rays are the same in those species with elongated mouths, as in 

 those whose mouths are unobtrusive, yet the difference in the shape of the perisomic 

 plates, those of the former being quadrilateral, and those of the latter mostly hexagonal, 

 frequently furnish good characters by which they may be easily distinguished fi-om each 

 other, even in the absence of all that portion of the animal abo^e the rays, and which 

 differs so materially in structure in diff"erent species. 



The excavations for the insertion of the rays are not so deep in this species as in P. 

 Icevis, but each had a similar transverse ridge running across the bevelled edge of the 

 excavation, with marginal stria;, as before observed in that species. One pm-pose of 

 these ridges appears to have been to secure the rays in their assigned position, and to 

 prevent the possibility of their shpping down fi-om the bevelled edges on whicli they 



rested. 



The perisomic plates are externally more convex tlian in the typical species, so that 

 the contour of the animal is somewhat petaloidal. 



