The Collaborative International Dictionary of English v.0.48:

Sulphur-bottom \Sul`phur-bot"tom\, n. (Zool.)
   A very large whalebone whale of the genus Sibbaldius,
   having a yellowish belly; especially, Sibbaldius sulfureus
   of the North Pacific, and Sibbaldius borealis of the North
   Atlantic; -- called also sulphur whale.
   [1913 Webster]