PKESTWICH — CRAG-BEDS OF STJFEOLK AND NOEFOLE. 



129 



species*. Alcide D'Orbigny f states that lie has dredged Bryozoa at 

 a depth of 160 metres off Cape Horn, and he considered the sea-bed 

 there to be formed of dead and living Bryozoa. In many places the 

 banks of Newfoundland are, in his opinion, covered entirely with 

 similar debris, GelleporaX being especially abundant. The conditions 

 most essential to the growth of Bryozoa, according to the same 

 experienced observer, are : — first, considerable depth of water ; 

 secondly, clear and limpid water ; thirdly, water constantly agitated 

 by waves and deep currents. 13 out of the 26 British species found 

 in the Coralline Crag belong to the genus LepraUa. A.ccording to 

 Dr. Johnston §, the greater number of these species are found in 

 deep water ; and Mr. Busk mentions that the Lejjralia ciliata has 

 been dredged at 45, the L. Peachii at from 110 to 147, the L. hya- 

 lina at from 4 to 40, and the L. Malusii at 48 fathoms. 



Amongst other common genera, of the Crag are Idmonea and 

 JRetepora, which may be considered essentially characteristic of 

 rather deep seas. The Escharce, again, live in deep water and among 

 strong currents. One extinct species of this genus, the E. monili- 

 fera, is the abundant and characteristic species of the Coralline 

 Crag. Another common species is the Bijlustra delicatula, now to 

 be found only in the seas of Australia and Manilla. The curious 

 globose genera Alveolaria and Fascicularia are extinct, and peculiar 



* Mr. Busk, however, has recently informed me that, from an examination 

 of the Bryozoa collected in the Mediterranean in the late voyage of the ' Porcu- 

 pine,' he believes that E. Beaniana occurs in that sea. He has also furnished 

 me with the following lists of Crag species now living in the Mediterranean 

 and Adriatic, or fossil in the Italian Pliocene beds : — 



C7'ag species occurring in the 

 Italian Pliocene. 



Lepralia ansata. 



„ Bowerbankiana. 



„ Brongniartii. 



,, ciliata. 



„ innominata. 



„ Malusii. 



,, mamillata. 



„ Morrisiana? 



,, Pallasiana. 



„ imicornis (tetragona, Beuss). 



„ violacea. 

 Retepora cellulosa. 

 Cupularia canariensis. 

 Membranipora andegavensis. 

 ,, oceani. 



„ Lacroixii. 



Biflustra delicatula. 

 Cellepora coronopus. 



„ ramulosa. 



„ scruposa. 



,, tubigera. 



t Annales des Sciences Naturelles, 1851, p. 295. 



J The curious Cellepora edax is often found covering, as a mantle, several spe- 

 cies of univalve shells. 



§ History of British Zoophytes. London, 1847. 

 VOL. XXVII. PART I. K 



Crag species living in the 

 Mediterranean. 



Salicornaria sinuosa. 

 Lepralia annulata. 



„ ansata. 



„ Brongniartii. 



„ ciliata. 



,, innominata. 



,, Malusii. 



,, Morrisiana ? 



„ Pallasiana. 



,, unicornis. 



„ violacea. 

 Eetepora cellulosa. 

 Cupularia cana:riensis. 

 Patinella patina. 



