INTRODUCTION. XXVU 



lowermost, by the dominance of Littorina littoralis, various 

 Bissow, especially Bissoa parva^ and Trochus cinerarius, 

 accompanied in the west by Trochus timbilicatus, and in 

 the south-west by Trochus Uneatus. 



A second region is the Circum-littoral or Laminarian 

 Zone; so called from the abundance of tangles or sea-weeds 

 of the genus Laminaria, that flourish in it around the 

 shores of Europe, On sandy ground these are replaced by 

 the Grass- Wrack or Zostera. Vegetable-feeding shell-fish 

 and naked Mollusca are exceedingly numerous in this space. 

 It is indeed highly productive of various types of animal 

 and also of vegetable hfe. Its usual vertical extent may 

 be stated as betvi^een low-water mark and fifteen fathoms. 

 In its lower portion the coral-weed, or Nullipora^ becomes 

 very abundant, and furnishes a ground often chosen by 

 fishes for their spawning haunts. The genera Lacuna (ex- 

 cept one species), Galyptrea^ Aplysia^ Scrohicularia and 

 Donax do not range, in our seas, below this belt ; and 

 Missoa, Chiton, Bulla, Trochtis, Mactra, Venus and Car- 

 dium have the majority of their British representatives 

 within its precincts. The sub-genus of Patella, called 

 Patina, is entirely confined to it. 



A third region is the Median or Coralline Zone, occu- 

 pying the space between fifteen and fifty fathoms. Sea- 

 weeds, properly so called, are scarce within it, and absent 

 from its greater portion, but much of it is clothed with an 

 animal-vegetation, so to speak, in the shape of corallines, 

 or hydroid- zoophytes. It abounds in shell-fish, and many 

 of our rarest and most valued kinds are procured from it. 

 In its upper portion Trochus ziziphinus and tumidus. Chi- 

 ton asellus, Acmeea virginea, Turritella communis, Venus 

 ovata and V. fasciata, Pecten opercularis, Modiola modi- 

 olus, the common form of Crenella, Pectunculus glycim^ris 



