374 TRANSATLANTIC SUB-REGION — ABYSS AL MOLLUSCA chap. 



Gould and Binney, in their Invertehrata of Massachusetts^ 

 enumerate 275 species (Cephalopoda, 6 ; Gasteropoda, 159 ; 

 Scaphopoda, 2 ; Pelecypoda, 108), of which 59 (Gasteropoda, 

 37 ; Pelecypoda, 22) are British. 



Among the characteristic genera are Urosalpinx^ Uupleura^ 

 Fulgur^ Ptyehatr actus ^ Nassa^ Crepidula ; Solenomya^ Mactra, 

 Oypricia^ Raeta^ Astarte^ and Yoldia. Our common Littorina 

 littorea appears to have been introduced into Nova Scotian 

 waters in about 1857, no previous trace of it occurring either in 

 literature or shell-heaps. Since then it has spread rapidly into 

 the Gulf of St. Lawrence, and also as far south as Newhaven, and 

 is said to be driving out the indigenous L. palliata from New 

 England shores.^ The debt has been repaid by the introduction 

 into British waters of the American clam ( Venus mercenaria L.), 

 which, according to the Manchester City Neivs of 23rd March 

 1889, was first observed in the Humber in 1864, and has steadily 

 increased up to the present time, when it bids fair to compete, 

 in those waters, with the familiar Cardium edide. 



Characteristics of Abyssal Mollusca. — Large shells appear 

 to be rare in the great ocean depths, and are usually very fragile ; 

 even moderately-sized specimens are far from common. The 

 only group in which species occur larger than the usual size is 

 the Nudibranchs, which are represented by at least one form 

 larger than an orange. 



It would seem that abyssal molluscs are much less active and 

 energetic than their brethren on the shores. This view is 

 favoured by the looseness of their tissues, which seem ill adapted 

 for prompt and vigorous action. The tenacious character of the 

 mud on the ocean floor must make rapid motion very difficult. 

 The shell itself is usually fragile and delicate, the upper layers 

 of arragonite being thin as compared with shallow-water species, 

 which makes the nacreous layer, when present, appear unusually 

 conspicuous ; in many cases the surface is characterised by a 

 peculiar iridescence or sheen. The colour in the shell of deep-sea 

 Mollusca is never very pronounced, and is often absent altogether. 

 Light pink and salmon, pale yellow and brown, are not uncommon. 

 If the colour is in pattern, it is usually in the form of necklaces 

 of spots, which sometimes coalesce into bands. With regard to 

 sculpture, stout knobs and powerfully buttressed varices, such as 



1 Amer. Nat. xx. p. 931. 



