_Squalodon._[150]—All the forms may be included in this genus, the so-called _Rhizoprion_ not being distinct. Dentition: _i_ ³⁄₃, _c_ ¹⁄₁, simple teeth of the molar series (premolars?) ⁴⁄₄, two-rooted molars ⁷⁄₇ = ¹⁵⁄₁₅; total 60. The double-rooted molars differ from those of _Zeuglodon_ in having the denticulations of the crown confined to the posterior border, or at all events much less developed on the front edge. Very little is known of the structure of these animals beyond the skull and teeth, fragments of which have been found widely distributed throughout the marine Miocene and early Pliocene formations of Europe, especially in the Vienna basin, many parts of France, and the Antwerp and Suffolk Crags. They have also been found in formations of corresponding age in North America and South Australia. A few isolated teeth have been met with in the cave-deposits of Italy, which, if contemporaneous with the beds in which they occur, indicate the survival of the genus into the Pleistocene period. _Family_ PLATANISTIDÆ. Under this heading may be placed three very singular genera, which, though differing considerably from each other, have several points in common, and do not altogether come under the definition either of the _Physeteridæ_ or the _Delphinidæ_, especially in the important character of the mode of articulation of the ribs with the dorsal vertebræ, the tubercular and capitular articulations, distinct at the commencement of the series, gradually blending together, as they do in most ordinary mammals. The cervical vertebræ are all free. The lachrymal bone is not distinct from the jugal. The jaws are long and narrow, with numerous teeth in both. The symphysis of the mandible exceeds half the length of the whole ramus. Externally the head is divided from the body by a slightly constricted neck. Pectoral limbs broad and truncated. Dorsal fin small or obsolete. Fluviatile or estuarine in habits. There are three distinct genera, which might almost be made the types of families, but it is probably more convenient to keep them together, only regarding them as representing three subfamilies.