As considerable misconception appears to exist regarding the type of the tenuirostral or vermiform family of the Diurnal Lepidoptera, we shall endeavour to illustrate this subject. Our concluding number is accordingly devoted to the genus _Polyommatus_ of Latrielle, and its subordinate types or sub-genera. These compose, what we have elsewhere defined, a natural and perfect group; (_North. Zool. 2, 288_) inasmuch as it has been tested by the analogies, and conformed by the representations, which it bears to innumerable others, both in the Annulose and vertebrated circles. According to this analysis, both _Lycæna_ and _Polyommatus_, strictly so termed, instead of being types either of families or sub-families, are of one and the same genus: which genus, moreover, is but the portion of the aberrant group of the _Theclinæ_. The typical forms of the genus _Erycina_, exclusively confined to Tropical America, constitute, in fact, the pre-eminent perfection of the family in question. As _Lycæna_ represents the _Nymphalidæ_, or sub-typical family of the Diurnal Butterflys, so is it the sub-typical form of the genus _Polyommatus_. Its geographic range is wide, being extended to the temperate latitudes of both hemispheres. The largest British species is that now figured, from the identical specimens mentioned by Lewin. * * * * * * [Illustration: POLYOMMATUS _Cassius_.] POLYOMMATUS Cassius. _Brazilian Blue._ ---- Family Erycinidæ. Sub-family Theclinæ. Genus Polyommatus (The typical sub-genus.) SUB-GENERIC CHARACTER. Wings entire, obtuse; the posterior rounded, particularly at the anal angle. Palpi covered and fringed with long hair; the last joint distinct and nearly naked. Antennæ with a lengthened, fusiform, spatulate club. Colour blue, beneath ocellated. ---- SPECIFIC CHARACTER. _Male: wings sub-diaphanous, blue, immaculate; whitish near the anal angle. Female: the disk of all white, with black spots and borders. In both sexes, beneath white, with blackish dots: those on the anterior wings transverse and confluent._ Pap. Cassius. (the female) _Cramer, pl. 23, fig. C. D._ ----