HOLOTHUROIDEA. 407 



based only on the occasional recognition of the microscopic plates or 

 spicules of the integumentary skeleton. The oldest remains which 

 can be certainly affirmed to be those of Holothurians are the micro- 

 scopic plates and spicules described by Mr R. Etheridge, jun., as 

 occurring in the Carboniferous rocks of Scotland. Some of these 

 have the form of rounded, perforated calcareous plates, about ^ inch 

 in diameter, associated with simple calcareous hooks or one-fluked 

 anchors, the shaft of the anchor having a perforation or " eye " at 

 its base (fig. 283, a and b). These plates and hooklets have been 

 referred to a special genus under the name of Achistrum ; and they 

 may be compared with the anchors and anchor-plates of the living 

 genus Synapta (fig. 283, c). Others have the form of circular, 

 generally concavo-convex, wheel-like plates, about j^q inch in diam- 

 eter, with a group of central pores and a series of marginal perfora- 

 tions (fig. 283, d). These resemble the wheel-shaped integumentary 

 plates of such living types as Thyonidium (fig. 283, e) or Chirodota, 

 and they may be provisionally placed in the latter genus. 



It is probable that investigations conducted with sufficient care 

 will ultimately show that the minute plates and spicules of Holo- 

 thurians have been more generally preserved in the fossil condition 

 than has been usually assumed to be the case. In the meanwhile, 

 with the exception of the remains above noted, no undoubted traces 

 of the former existence of Holothurians have been found until the 

 Jurassic rocks are reached. Here, the occurrence of minute wheel- 

 like plates, resembling the " wheels " of the recent Chirodota has 

 been recorded. Pocta has described plates of Psoitis from the 

 Chalk of Bohemia, and Schlumberger has recently found the 

 characteristic armature of Synapta, Chirodota, and Thyonidium in 

 the Middle Eocene of the Paris basin ; while Chirodota occurs in 

 the Pliocene of Northern Italy, and plates belonging to Psoitis have 

 been found in Post-tertiary deposits in Bute. 



