56 THE LIFE OF THE MOLLUSCA 



examples of Chceto derma off Nova Scotia at 1,250 

 fathoms. A Gastropod (Stylifer brychim) was brought 

 up in the South Atlantic from a depth of 2,650 

 fathoms ; a Nudibranch (Bathydoris abyssonim) in 

 the Pacific Ocean from 2,425 fathoms; and some 

 Pelecypods also in the Pacific from 2,900 fathoms ; 

 while a Scaphopod (Dentalium leptoskeles) was ob- 

 tained from 2,600 fathoms off the south coast of 

 Australia. Cuttlefishes have been taken from between 

 2,000 and 3,000 fathoms, but some uncertainty 

 attaches to the records in their case, since none of 

 the forms obtained at these depths were distinctly 

 dwellers on the sea-floor. 



No hard and fast line can be drawn between the 

 several zones above enumerated, and some species 

 range over more than one of them ; nevertheless, the 

 prevailing forms in each serve to distinguish them. 



Conspicuous and most abundant among the ocean- 

 swimming molluscs are the Sea- Butterflies (Plate 

 XXVI. , Figs. 1-16), formerly grouped in a class, as 

 Pteropoda, but now recognized as highly specialized 

 Tectibranchs — those with shells branching off from 

 the Bulla-like section, while the shell-less ones are 

 more nearly related to the Sea-Hares (Aplysiidae). 

 Less abundant are the Heteropoda (Plate XXVII. , 

 Figs. 17-22), now known to be free-swimming Tsenio- 

 glossa, although they, too, were once classed apart. 

 A Nudibranch (Phyllirrhoe) similarly specialized, 

 and the Tsenioglossate (Ianthina), with countless fry 



