BRACHIOPODA 185 



there being only about 120 living species. Many forms 

 occur more abundantly where the sea-bottom is rocky, or 

 stony, or formed of corals, than where it is soft or muddy ; 

 frequently they are much localised, being found in enor- 

 mous numbers at one spot, whilst, in the adjoining areas, 

 they are sparsely distributed. About half of the existing 

 species are found at depths of less than 100 fathoms, and 

 several of these do not extend beyond this limit. Below 

 150 fathoms they soon become comparatively rare, but a 

 few species {e.g. Terebratulina wyvillei) occur down to 

 2900 fathoms. Some genera, as for instance Lingula, do 

 not occur below a depth of a few fathoms. In the 

 Littoral zone (see p. 252) Brachiopods are less numerous 

 than elsewhere, only 17 species occurring in it. The 

 Laminarian zone contains 46 species, of which 15 (in- 

 cluding 5 species of Lingula) are confined to it. In the 

 zone of Nullipores there are rather fewer forms than in 

 the preceding one. The zone of Brachiopods and Corals 

 is, as its name suggests, the richest in Brachiopods, con- 

 taining no less than 58 species or varieties. In the Abyssal 

 zone 30 species have been dredged, but the greater number 

 come from the upper part of the zone ; the Inarticulata are 

 poorly represented here. Geographically, the Brachiopoda 

 which live in comparatively shallow water are distributed 

 in provinces, agreeing generally with the Molluscan pro- 

 vinces (p. 251). These are characterised by the presence or 

 abundance of certain species, the ranges of which are 

 determined mainly by climate. With one or two excep- 

 tions all the species found in the Northern Hemisphere 

 are distinct from those in the Southern. Amongst the 

 forms limited to the cold northern regions may be 

 mentioned Rhynchonella psittacea&nd Magellania septigera, 

 whilst R. nigricans and M. venosa are found only in cold 



