Although of sombre colours, this butterfly is imposing in its size, and highly interesting. It is, in fact, that form which is _pre-eminently typical_ of the whole of the Latriellian _Papilionidæ_. It seems to be common throughout India and particularly so in Java, from whence we possess several specimens. Dr. Horsfield's elaborate work has furnished us with figures of the caterpillar and chrysalis. In now first defining some of the natural groups of the modern Genus _Papilio_, it may be as well to state that we give the results of minute analysis, the details of which we hope to lay before the public on a future occasion. In regard to the nomenclature, we have adopted the following principles, suggested to us by a scientific friend of no ordinary authority, as tending to facilitate recollection, without altering well known specific names. Each genus or sub-genus will be named after that species which is its peculiar type; and the new specific name of this species will imply one of its supposed natural analogies. Thus the sub-genus _Protesilaus_, derives its name from the typical species, while the specific name, now proposed of _Leilus_, points out the analogy of the group to _Urania_ Fab. The sub-genus we now define, as being in our estimation, pre-eminently typical, retains the name of the genus. It seems also a geographic group, since all the species yet discovered belong to the old world. * * * * * * [Illustration: CARACOLLA _acutissima._ _Two-toothed Disk-Snail._] CARACOLLA acutissima, _Two-Toothed Disk Snail_. ---- Class Mollusca. Order Phytophages. _Swains._ GENERIC (?) CHARACTER. _Animal_ with four tentaculæ, the lower pair very short; _Shell_ discoid, greatly depressed; the spire but slightly raised above the body whorl; aperture large, oblique, angulated; the lower portion generally dentated; the margin thickened and reflected. ---- SPECIFIC CHARACTER. _Shell imperforate, with the spiral whorls flattened obliquely, the body whorl acutely carinated, and convex beneath: outer lip reflected; with from 1 to 2 tuberculated teeth near the extremity_. Caracolla acutissima. _Lam. Syst. 6. p. 2. p. 95. Knorr. vol. 4 pl. 5. f. 2. 3._ _Encycl. Meth. pl. 462. f. 1. a. b.?_ Helicodonta. _A. de Fèrrusac. pl. 58. f. 2._ Helix caracolla. _Guerin. Iconog. du Règ. Anim. Mol. pl. 6. f. 1._ ----