HOLOTHUROIDEA. 305 



but differs from all known genera of this order in having 

 only four pseud-ambulacral areas, in having been free and 

 not provided with any peduncle, and in having the calycine 

 plates ornamented with prominent tubercles. 



ORDER VII. HOLOTHUROIDEA. 



The last order of the Echinodermata is that of the Holo- 

 thurians or " Sea-cucumbers," in which the ~body is vermiform 

 or slug- shaped, and the calcareous matter secreted ~by the integu- 

 ment is reduced to scattered spicules (Synapta), or rarely is pres- 

 ent in the form of imbricated scales (Psolus). 



As might have been expected from the generally soft 

 nature of their integuments, the Holothuroids are hardly 

 known as fossils, and they merely require to be mentioned 

 here. The only remains referred with any probability to 

 this order are certain calcareous spicula which have been 

 found in deposits as old as the Carboniferous, and also in 

 strata of both Mesozoic and Tertiary age, and which have 

 been regarded as belonging to forms related to Synapta, while 

 the shield of a species of Psolus has been found in Post-Ter- 

 tiary deposits in Bute. 



It seems very probable, however, that the more universal 

 use of the microscope by palseontological observers will re- 

 sult in the discovery of the anchor-shaped spicules and 

 wheels of the Synaptidce in all the more modern, at any rate, 

 of the formations which compose the crust of the earth. 



LITERATURE. 

 GENERAL. 



1. " Die Klassen und Ordnungen des Thierreichs." Vol. ii., Strahl- 



enthiere. Bronn. 1860. 



2. " Ueber den Bau der Echinodermen." J. Miiller. Abhandl. der 



Berl. Akad. 1853. 



3. " Lethaea Geognostica." Bronn. Third edition. 1851-56. 



4. " Index Palseontologicus." Bronn. 1850. 



5. " Catalogue of British Fossils." Morris. 



ECHINOIDEA. 



6. " Monographies d'Echinodermes vivants et fossiles." Louis Agassiz. 



1838-41. 

 VOL. I. U 



