Cetaceans. 19th-century artwork of (from top) four cetaceans: a dolphin (Delphinus delphis), a narwhal (Monodon monoceros), a sperm whale (Physeter macrocephalus), and a Greenland or bowhead whale (Balaena mysticetus). The narwahl's helical, hollow tusk is an elongated canine tooth. Dolphins feed on squid and fish, narwhals on seabed flatfish, sperm whales mostly on squid, and bowhead whales on plankton filtered using baleen plates. Artwork from 'Encyclopaedia Metropolitana', an encyclopedia published in London between 1817 and 1845. Thirty volumes were issued in 59 parts, with 22,426 pages and 565 plates.