Not supposing the _lateralis_ to belong, strictly speaking, to either of these genera, and with a view to ascertain its real nature, we obtained permission from the Academy of Natural Science, to open a specimen belonging to their cabinet, and which was brought from the Ohio by Mr. J. Speakman. The result corresponded with our most confident expectations, showing that the number of its vertebræ is greatly inferior to that of the Proteus, and corresponding with that of the Tritons; and that the pseudo ribs were in an entire series, somewhat superior in proportional length and perfection of form to those of the Proteus, and resembling those of the Triton. It has, therefore, a far more close alliance with the genus Triton, than with any other yet established. Several animals have been described, to which it is more closely related by the character of the persistent branchia, than it is to the well-known types of the genus, of which the branchia disappear at the age of puberty. Of such animals the following may be instanced: The _Axolotl_ of Mexico. Siren pisciformis of Shaw. Gen. Zool. The _Tetradactyla_ of Lacepede in the Ann. des Mus. vol. x. The _Siren Operculée_ of Beauvois in Philos. Trans. of Phila. vol. iv. And possibly also, the _Proteus Neo Cæsariensis_ of Professor Green.--Jour. A. N. S. vol. i. These four or five species might with propriety be separated from the genus to which they are referable in the present state of the system, and placed in a separate genus, the external characters of which will be the same as those of Triton, with the exception of the persistent branchia. Its proper station will doubtless be intermediate between Triton and Proteus, but far more closely related to the former. It may be proper to mention in this place, that the generic name _Triton_, was applied by Laurenti to the Newts, long before Montfort made use of it in Conchology to designate the war conch of the ancient Romans, and of the present inhabitants of Madison's Island. We are indebted to Dr. Richard Harlan, for the following anatomical observations, on this singular animal.