On the left side of the entrance is a case containing a stuffed specimen, a skeleton, and several skulls of the Susu, or Fresh-water Dolphin (_Platanista gangetica_) of the rivers of India, and also of the Dolphins of the Rio de la Plata (_Pontoporia blainvillei_) and of the River Amazon (_Inia geoffroyensis_). Among the specimens fronting the visitor as he enters the room, one of the most interesting, on account of its remarkable dentition, is the Narwhal, or Sea-Unicorn. It has only two teeth, which lie horizontally in the upper jaw. In the female both remain permanently concealed within the bone of the jaw, so that this sex is practically toothless; but in the male, while the right tooth remains similarly concealed and rudimentary (as shown in the specimen, by removal of part of the bone which covered it), the left is immensely developed, attaining a length equal to that of half the entire animal, and projecting horizontally from the head in the form of a long, straight, tapering and pointed tusk, spirally grooved on the surface. In rare cases, as in the skull exhibited near the skeleton, both teeth are fully developed, and it is noticeable that in such specimens the direction of the spiral is the same in both tusks. [Sidenote: Sperm-Whale.] To the right of the entrance is placed a specimen of the bony framework of one of the most colossal of animals, the Cachalot, or Sperm-Whale (_Physeter macrocephalus_), fig. 32, prepared from an old male cast ashore near Thurso, on the north coast of Scotland, in July, 1863, on the estate of Captain D. Macdonald, R.E., by whom it was presented to the Museum. Upon one side of this skeleton has been built the model of the external form of the animal. The Sperm-Whale is the principal source of supply of sperm-oil and spermaceti: the former being obtained by boiling the fat or blubber lying beneath the skin over the whole body. The latter, in a liquid state at the ordinary temperature of the living animal, is contained in cells which fill the immense cavity on the top of the skull. This Whale, which feeds chiefly on Cephalopods (Squids and Cuttle-fishes), but also on Fishes, is distributed throughout the warm and temperate regions of the Atlantic and Pacific Oceans, and sometimes enters British waters. In order to render this skeleton more instructive, the names have been attached to the principal bones, thus enabling the visitor to trace at a glance the extraordinary modification from the normal mammalian form the huge skull of this species has undergone. [Illustration: FIG. 32.--AN OLD MALE SPERM-WHALE OR CACHALOT. Skeleton and outline of animal: _b_, nostril or blowhole; _p_, rudimentary pelvic bone. Length of specimen 54 feet.] [Sidenote: Whalebone Whales.]