The genus Prodelphinus has, like Delphinus, a distinct beak; but it has not the grooved maxillaries. No other character of importance appears to separate it from Delphinus.
The genus consists of some eight widely distributed species, which are none of them large Dolphins.
Lagenorhynchus has the following assemblage of characters:—Head with short, not very distinct beak. Dorsal and pectoral fins falcate. Teeth small, twenty-two to forty-five in each half jaw. Vertebrae ranging in number from seventy-three to ninety-two. Pterygoid bones either in contact or separate. There are fifteen or sixteen pairs of ribs, of which six are two-headed. Of this genus Mr. True allows eight species, which have been increased by a ninth since the publication of his "Revision."[[250]]
Two species of Lagenorhynchus are known from our coasts; the rest are mainly southern in range. The British species are, firstly, L. albirostris, a Dolphin of some 9 feet in length. It has a large number of vertebrae, ninety-two in number. L. albirostris is a rare species, the first record of its occurrence on these shores being in 1834. Since that date some eighteen individuals have been shot or stranded on the shores of the British Isles. The second British species, L. acutus, differs in colour from the first. As in the last, the upper parts are black and the under parts white; but in L. acutus there is also a stripe on the flanks, brownish in colour. It has fewer vertebrae, not more than eighty-two.
The next genus of Dolphins, Sotalia, is characterised by—Teeth tolerably large, twenty-six to thirty-five. The vertebral formula is C 7, D 11 or 12, L 10 to 14, Ca 22. The pterygoids are not in contact in the middle line. It has a distinct beak.
Of this genus there are some six species (the exact number, as in so many other genera, cannot be positively asserted), most of which are fluviatile or estuarine in habit. They are also on the whole characterised by their pale, if not actually white, coloration. S. sinensis of the Amoy is white with pinkish fins. Sotalia guianensis is American as its name denotes. It is figured by van Beneden as of a pale brown colour. It is very abundant in the Bay of Rio de Janeiro, and has the reputation of being a friend of man like some other Dolphins. The natives hold that it will bring to shore the bodies of drowned persons. The most singular species of the genus is that recently described by Professor Kükenthal as S. teüszii.[[251]] This animal is purely fresh-water, being found in
the Camaroon river, where it is extremely rare. The nostrils (blow-hole) are prolonged into a snout-like process, a fact which is of interest in connexion with the assertion that in Balaenoptera the blow-hole is puffed out during spouting. What is temporary in the Rorqual appears to be permanent in the Sotalia. More remarkable still, perhaps, is the assertion that it is a vegetable-feeding Dolphin. This is not a mere assertion except that it may not apply universally; for in the stomach of a specimen nothing but vegetable débris was found. But in the stomachs of other Whales (e.g. Rhachianectes) vegetable matter has also been found, which may perhaps have been taken in accidentally with the food.
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.