56 THE LIFE OF THE MOLLUSCA 
examples of Chétoderma off Nova Scotia at 1,250 
fathoms. A Gastropod (Stylifer brychius) was brought 
up in the South Atlantic from a depth of 2,650 
fathoms; a Nudibranch (Bathydoris abyssorum) in 
the Pacific Ocean from 2,425 fathoms; and some 
Pelécypods also in the Pacific from 2,g00 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 
XXVIL., 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 (Aplysiidz). 
Less abundant are the Heteropoda (Plate XXVII., 
Figs. 17-22), now known to be free-swimming Tzenio- 
glossa, although they, too, were once classed apart. 
A Nudibranch (Phyllivrhoé) similarly specialized, 
and the Tzenioglossate (Ianthina), with countless fry 
