XXVlll INTRODUCTION. 



and Nucula nucleus, are characteristic testacca, and in its 

 lower half Solen pellucklus, Pecten varius^ Modiola modi- 

 olus, Dentallum and Mactra eUiptica. It is marked more 

 by the peculiarities of its species than by the exclusive 

 presence of genera. 



The fourth region is the Infra-median, also called the 

 region of deep-sea corals, since in our seas (though not in 

 those of the tropics) the principal stony corals, whether of 

 zoophytic or bryozoan production, are procured from it. 

 Its most characteristic portions, within the British area, 

 are in the extreme north. There appear, however, to be 

 very few species of Mollusks peculiar to it within our seas. 

 Those that are found are, for the most part, of rather 

 small dimensions, and remarkable for being of dull or pale 

 colouring. 



Beneath this zone is the Abyssal Region, which can 

 scarcely be said to be developed within the British area. 



The greater part of the floor of the British seas is occu- 

 pied by the Coralline or Median zone. The Infra-median 

 belt is of less, though considerable, extent. The Abyssal 

 or deep-sea region occupies but an insignificant portion of 

 their area. Hence the very small number of our Mollusks 

 that can be fairly regarded as characteristic abyssal species. 

 The coast-line of the British Islands is exceedingl}' varied, 

 in consequence of the numerous formations of various 

 mineral constitution and geological age of which the land 

 is composed. A corresponding variety in the species, and 

 abundance of distinct specific types, are the result, and our 

 Molluscan fauna, in consequence, becomes an exceedingly 

 rich one. The very shallowness of a considerable extent 

 of our seas is in favour of their richness, when combined, 

 as it is, with considerable variety of sea-bed. An inspec- 

 tion of hydrographical charts will readily illustrate this 



