poteriocrinid^.-poteriocbinus. 87 



Synonymes and References. 

 Cladocrinites pentagonus. — Austin, Ann. 8f Mag. Nat Hist. Vol. \\. p. 198. 

 Formation and Locality. 



Altered beds of mountain limestone, Clevedon Bay, Somersetshire. 



The basal and succeeding ray joints of this species of Poteriocrinus articulate by such 

 broad surfaces as to appear like a third or fourth series of perisomic plates resting on the 

 pentagonal ray bearing plates. The second or upper joint, of each of the primary rays 

 is pentagonal, and equally broad at its attachment, as that on which it rests. The salient 

 angles of these pentagonal pieces point upwards, so that their upper and outer sloping 

 edges give support to the lesser rays. No further bifurcation takes place in the three 

 out of the five groups of rays, but each of the remaining two, which are placed one on each 

 side of the open space formed by the introduction of the inter-radial plates, as represented 

 in platel 1 ,fiig.2 b & 2c, have an additional bifurcation, as if designed to guard the interval 

 thus left, and to complete the circle, when the rays and tentacula, were spread out in 

 quest of prey. On reference to the figures before alluded to, it will be seen that the 

 open space caused by the introduction of the inter-radial plates, produces a corresponding 

 interval in the net like circumference of the rays, when spread around, through which 

 the minute creatures, which in all probability, formed in part the food of this crinoid, 

 would have escaped, had it not been for the increase in the number of rays, at the exact 

 point where some such contrivance was necessary to complete the apparatus for securing 

 its prey. 



Although we have not observed, in any other species of crinoid a similar partial increase 

 in the number of rays, placed as it were to guard the passage made by the interval 

 where the inter-radial plates occur, it is probable that additional rays do exist in several 

 other species, in which there is a wider interval between the rays at one part of the 

 circumference, than iii the other four. 



We have dwelt rather much on this subject, because a more interesting fact, or one 

 that goes further to establish the manner in which a seeming irregularity of structure 

 may be made subservient to a perfect adaptation of a means to an end, is not to be found, 

 even among the beautiful and once abounding race of animals, which constitute the 

 object of our present inquiry. 



The oral tube, PI. 11. figure 2 e, appears to be formed of five vertical bands of plates, 

 but the pieces are not marked with elevated striae, as in most other species of Poterio- 



