In 1886, H. J. Kolbe, in his "Ueber die Stellung von Platypsyllus im System" (Berlin entom. Zeitsch., xxx., 1886, pp. 103-105), discusses the subject, without any new evidence, however. He concludes that most of its characteristics relate it to the Corrodentia, and particularly to the sub-order Mallophaga, in which it has its closest kinship in Liotheidæ. The remarkable tripartite mentum he thinks should not be compared with the bipartite mentum of Leptinus, and calls attention to the fact that in Ancistrona in Mallophaga it is also trilobed. The above are the more important papers on the subject, though the insect has been referred by other authors to both Neuroptera and Orthoptera. CHARACTERS OF PLATYPSYLLUS. [Illustration: LARVA OF PLATYPSYLLUS CASTORIS--DORSAL VIEW.] [Illustration: PLATYPSYLLUS CASTORIS.] Where the characters of the image have been so often described, it is unnecessary to refer to them in detail, and I will only call attention to the more striking structural features and to some omissions by, or differences between, previous authors. A glance at the illustrations which I have prepared will show the prevailing characteristics of this interesting creature, its general ovoid and flattened form, and more particularly the flattened semicircular head. Dorsally, we notice the rather prominent occiput fringed behind with short and broad depressed spines or teeth which form a sort of comb, the prothorax trapezoidal and but very slightly curved, with side margins strongly grooved. There is a very distinct scutellem, and the two elytra are rounded at the tip and without venation. Hind wings and eyes are both wanting. The abdomen shows five segments, each with a row of depressed bristles. On the ventral surface we find among the more curious characteristics, first the antennæ; these were originally described by Westwood as three-jointed, the club being annulated. Le Conte could not distinctly make out the number of annular joints upon this club, though he thought he detected seven, which made nine joints to the whole antenna. The club is received in the deep cup-shaped excavation of the second joint. Horn thought he detected a division of the second joint, and resolved but six segments in the club, making also nine joints to the whole antenna, but in a somewhat different fashion from Le Conte. Westwood's figure shows eight annuli to the club. He failed to find any trace of the mandibles, but Le Conte described them as small, flat, subquadrate, with the inner side deeply crenulate, and resembling those of _Corylophus_; the stipes well developed, and biarticulate. Horn could not entirely make out the mandibles as described by Le Conte, and rather concluded that what Le Conte described is really one of the granules which occur behind the labrum. He considered that the piece could hardly be even an aborted mandible, because of its diminutive size. [Illustration: YOUNG LARVA.]