HOLOTHURIOIDEA 37 



smallest plates resemble bits of perforated card-board, and the 

 largest and most solid are formed of a repetition of similar 

 laminae. In a few membranous structures, minute spicula, 

 curved, bi-hamate, or anchor-shaped, are met with. They are 

 always composed of carbonate of lime ; but owing to their 

 porosity, fossil examples are commonly impregnated with 

 earth, or pyrites, or silica, and form bad subjects for micro- 

 scopic investigation. Without, however, losing their organic 

 structure, the fossil Echinoderms exhibit a cleavage like that 

 of calcareous spar, by which the smallest ossicle of star-fish or 

 Crinoid may be recognised : this peculiarity is most strikingly 

 obvious in the great spines of the Cidaris (fig. 8, 3), or the 

 enlarged column of the " pear Encriiiite " (fig. 6, 7). Examples 

 of the latter may be seen which had been crushed when 

 recent, and before the sparry structure was superinduced. 



Order 0. — Holothurioidea. 

 (Sea-Cucvmbers, Trepang.) 



Char. — Body vermiform ; integument flexible, with scattered 

 reticulate calcareous corpuscles, or beset with small 

 anchor-shaped spicula. 



The Holothurioid order presents scarcely any examples 

 likely to be met with in a fossil state, except the genus Psolus, 

 of whose imbricated shield a fragment has been found by Mr. 

 Richmond in the northern drift of Bute. Count Minister has 

 figured the microscopic plates, apparently of a Holothuria, from 

 the chalk of Warminster ; and the anchor of a Synapta from a 

 still older formation, — the upper oolite of Bavaria. Micro- 

 scopic observers will doubtless meet with many such detached 

 plates and spines when searching for Polycystinese and other 

 Pdiizopods in the oolitic and cretaceous strata ; but it is scarcely 

 probable that the order has dated far back in time. 

 * Beitrage, heft 6, 1843. 



