78 POTERIOCRINID.E. POTERIOCRINUS. 



All the plates of the adult animals of this species appear to be covered with fine granulae, 

 but the younger specimens have only faint indications of a granulated surface. Each 

 division of the dorso-central plate has a deep notch on its lower terminal angle, the 

 combined effect of this, when the plates are in their proper positions, is to produce a 

 pentapetalous aperture into the cavity containing the digestive organs, as represented 

 in plate 9, fig. 3 e. 



The column is circular, large and long, with a pentagonal shaped canal running through 

 its upper portion, and gradually becoming circular as it descends towards the base. 



This species of Poteriocrinus in its short tumid plates, its broad surface of articulation 

 for the rays, and contracted cavity for the digestive organs, bears some analogy to the 

 Apiocrinus elKpticus of the chalk, and the A. elongatus of the oolite. 



Much of the Mountain limestone of Derbyshire is almost wholly composed of the broken 

 and disjointed columns of this species of Poteriocrinus, and yet amidst these countless 

 fragments of columns only a few bodies have been discovered. The probable 

 conjecture as to the cause of this deficiency of the bodies of this animal, which we 

 know nmst have existed, for eyary column supported a body containing the digestive 

 organs, is, that the predaceous fishes of the period, preyed on the superior portions and 

 rejected the other parts, and by this means a gradual accumulation of columnar fragments 

 took place, vihile the plated bodies were crushed by the bony palates of their destroyers. 

 This is the more probable as detached plates are as rarely found as complite bodies, and 

 which must have been scattered through the matrix, had the animals been left to the 

 patural and gradual progress of decay. 



The ornamental marble from Derbyshire, and mostly knovm by the name of Fucrinital 

 Marble, owes its chief beauty to the sj'^mrnetrical columns of this species of Poterio- 

 crinus. 



5. Species. Potekiocrtnus plicatus. [Aur>tin.) 

 PI. 9. fig. 4«, toAf. 



Definition.— The arrangement of the plates surrounduig the digestive organs agree with 

 the typical species, but they are much wider in proportion tu their lieiglit ; angles of the 

 plates depressed, so as to form series of lozenge ohuped and triangular iridentation.saround 

 the body ; ray articulations semicircular and occupying about a tiiird of the ray bearing 

 plates ; column large but thin jointed. 



