114 PROCEEDINGS OF THK M ALACOLOGICAL SOCIETY, 



up of three elements: (1) a nortliern element, consisting of species 

 wliicli may be supposed to have spread southwards from Arctic or 

 sub- Arctic latitudes; (2) a southern element, consisting of species 

 which have spread in the reverse direction from more southern 

 latitudes; (3) an element wliich is probably indigenous in our own 

 and neighbouring temperate seas. 



Perhaps the most striking way of bringing out this point is to 

 instance particular genera which happen to include species of both 

 northern and southern origin. Of Littorina, for instance, we have 

 four species on our shores, three of which are northern and one 

 southern in origin : littorea, L., a northern form, ranges from the 

 Wliite Sea and Mourmane coast to Lisbon and the Straits of Gibraltar ; 

 rudis^ Mat., from the Glacial Ocean to Southern Spain'; obtusata, L., 

 extends from the AVhite Sea, Finraark, and Iceland to South Spain 

 and South France, but not further east in the Mediterranean. All 

 these three species are found on the east coast of North America. 

 Z. neritoides, L., on the other hand, is a markedly southern species, 

 ranging from the Canaries and Madeira to North Britain. Oi Acmaa 

 we iiave two species, one of markedly southern, the other of ec^ually 

 clear northern origin ; A. virginea, ^liill., ranges from St. Helena, the 

 Azores, and Madeira to North Norway; A. testiiduialis, Miill. 

 (a thoroughly Arctic form), occurs from Nova Zembla, North Labrador, 

 Greenland, and all Arctic seas to the Yorkshire coast on this side of 

 the Atlantic, and to New Jersey on the other. Emarginula is 

 represented by three species, each of which appears to belong to 

 a different fauna; E. crassa, J. Sow,, is a northern form, curiously, 

 as it seems, absent from tlie eastern coasts of Britain, and found in 

 littoral and shallow waters no further south than Dublin Bay ; 

 £. Jissura (L.), with a range from Finmark to the Canaries, seems 

 characteristic of the temperate fauna, while E. eofitca, Schum., is 

 a strictly southern form, ranging from the Mediterranean and South 

 Spain to the Dorset coast, but no further north. The same point 

 may be illustrated in other of the genera occurring on our shores, 

 e.g. Modiolaria, CreneUa, Rissoa, Scala, C'aUiosfoma, and Lxmatia, of 

 wiiicli latter genus pallida., Brod., mnntagui, Forbes, and affinis, 

 Gmel., are northern forms, alderi, Forbes, belongs to the temperate 

 fauna, while catena (da Costa) and sordida, Phil., are of southern 

 origin. 



The following members of the British marine fauna rank as 

 ' northern ' species (the list has no pretensions to completeness, and 

 Nudibranchia and Cephalopoda are not included) : — 



*-\X%Tonicella marmorea (Fabr.).- *-\XNuculana tenuis (Phil.). 



tr. rubra (Lowe). *Liinopsis mirita (Broc). 



*\Craspedochihis albtis (L.). [i.%Modiolus modiolus (L.). 



' Unless we unite rudis, Mat., and so.rntilis, Oliv., in which case the range 

 extends all over the Mediterranean, Adriatic, and Black Seas. 



'^ R. McAndrew is said to have dredged this species at Carthagena in 5-10 f., 

 which seems improbable. 



