_Steno_ comes near _Sotalia_, and Dr. Blanford has transferred to it (under the one name of _Steno perniger_) the two species, _Sotalia gadamu_ and _Sotalia lentiginosa_. It is, however, to be distinguished from _Sotalia_ by the following characters:--Teeth large and few, twenty to twenty-seven on each side of each jaw, with furrowed surfaces to crowns. Vertebrae C 7, D 12 or 13, L 15, Ca 30 to 32. Pterygoids in contact. There are but two species apparently (not counting Dr. Blanford's). _Tursiops_ is not a very easily definable genus. These are its chief features:--Teeth large, twenty-two to twenty-six in number in each half of each jaw. Vertebral formula C 7, D 12 or 13, L 16 or 17, Ca 27. Pterygoids in contact. Beak distinct. Some five species are allowed; but it seems to be difficult to differentiate the others from _Tursiops tursio_. This, the best-known form, is quite or nearly world-wide in range, and occurs, though not abundantly, on our own coasts. Mr. True has observed that the eyelids of this Whale, which is largely hunted on the American coast, are as mobile as those of a terrestrial mammal. The name "tursio" is derived from Pliny. Belon would also derive from this word the French vernacular "marsouin." The latter term is sometimes regarded as a corruption of "Meerschwein," but it would seem to be more probably derivable from "marinum suem," from the Latin direct. _T. tursio_ has the back black to lead-colour; the under parts white. In the reputed species, _T. abusalam_, from the Red Sea, the back is a dark sea-green. _T. tursio_ reaches a length of 12 feet, but is more usually smaller. The genus _Tursio_ must be carefully distinguished from {380} _Tursiops_. It has no dorsal fin, the teeth are small and numerous (forty-four), and the pterygoids are separate. There are two species, _T. borealis_ and _T. peronii_, the former being northern and the latter more widely spread. The genus _Cephalorhynchus_ has for its chief characters the following:--Teeth twenty-five to thirty-one, small and sharp. Pterygoids widely separated. Dorsal fin not falcate, but triangular or ovate in form. Beak not well marked off from the head. The species of this genus are all southern in range; four are perhaps to be allowed. FAM. 3. PLATANISTIDAE.--This family of Odontocetes may be distinguished from the Dolphins by the following assemblage of structural features:--Cervical vertebrae all free, and each one of some length (for a Cetacean). Jaws long and narrow, with a considerable length of symphysis. Teeth very numerous.