THE EUROPEAN SEAS. 



183 



its characters precisely defined, and usually before 

 the characteristic species of one region has declined 

 to the numerical minimum of individuals, its suc- 

 cessor has appeared ; at first scarce, but when in 

 its true or proper region, as abundant as its prede- 

 cessor had been. The characteristic Fuci are Dictyo- 

 menia vohtbilis, Sargassum salicifolium, Codium 

 bursa, C.flabelliforme, and Cystoceira. The rare and 

 curious Hydrodictyon umbilicatum was procured in 

 this region. Urchins are abundant, and Comatulce. 



The Bivalves are about sixty, of which thirty- 

 three have their maximum here. 



The Univalves are about ninety, with about 

 forty having their maximum here. 



Terebratula detrancata and cuneata make their 

 appearance here, the latter being most abundant. 



JRetepora cellulosa abounds ; and Myriapora trun- 

 cata, with Cell aria ceramioides, are characteristic. 

 The finest sponges of commerce are taken from 

 this zone. Nucula emarginata serves generally to 

 mark this depth. 



The Fifth Region extends from thirty-five to 

 fifty-five fathoms; its plants are Bytiphlcda tinc- 

 toria and Chrysimenia uvaria. Dictyomenia volubilis, 

 which gives a marked character to the preceding 

 zone, becomes scarce here. The sea-bed is composed 

 of Nullipores, and is shelly. The Testacea most 

 generally distributed are Pecten opercidaris, Turri- 

 tella tricostata, and the most abundant in individuals 



