Having recently examined the European Cossus (Xyleutes) ligniperda, the generic differences between Cossus and Prionoxystus impressed me quite forcibly. The Cossidæ or Teredines of Hübner (Grote, Proc. Am. Phil. Soc., Nov., 1874), form a sub-family group of the “Spinners” or Bombycidœ, and are generally characterized by the wood-eating habit of the larvæ, which are provided with powerful mandibles for the purpose. The moths are generally of a clumsy build and are usually of a gray color, resembling the bark of trees on which they rest, and are more or less reticulated with darker shade lines, peppered and spotted so as to give the idea of protective imitation. The ocelli are wanting and the tongue is rudimentary. In the gents Cossus, and in most of the genera of the group, the vestiture of the body is close, thick and hairy. Although gray is the prevailing color, some Australian species have the hind wings especially of a reddish-brown hue. The European Zeuzera Aesculi is white, spotted over wings and thorax with steely blue. The North American genus Prionoxystus differs at first sight by the sparse, thinly laid on, scaly vestiture.