576 TRANS. ST. LOUIS ACAD. SCIENCE. 



ornament ; in the sub-obsoleteness of the piliferous spots, and the 

 nonconspicuity of those on joint 11 ; in the subdorsal region being 

 pale and the stigmatal region dark. All these characters, together 

 with the third pair of prolegs, belong to pometaria as distinguished 

 from vernata. From pometaria the specimen differs in the nar- 

 rowness of the pale lines, and of the medio-dorsal dark line ; and 

 while it may be an aberrant specimen of this last, it cannot possi- 

 bly be vernata. The probability is that it belongs to another spe- 

 cies entirely, as there are several other Geometrid larvae that feed 

 on Elm and Apple. 



I speak with confidence because I have reared both the Canker- 

 worms from the egg and have a vivid knowledge of their larval 

 characteristics, some of the most important of which are not stated 

 in Dr. Packard's description of vernata. 



Though the word either occurs with awkward frequency in the 

 diagnosis referred to, it should nevertheless occur still more often, 

 until all the opposite and heterogeneous characters in the left-hand 

 and right-hand comparative columns, as published by me (pp. 

 274-7), are brought together. 



The mixed genus thus obtained is cited by Dr. Packard, in the 

 Introduction of the work, as an instance of the small classifica- 

 tory value of the number of legs in larvae ; which is about as con- 

 vincing as it would be to a botanist, if some one were to throw 

 Caltha and Anemopsis together because of their superficial resem- 

 blance, and then cite the mixed genus as evidence of the classifi- 

 catory worthlessness of the important characters which cause 

 those genera to be placed in different families. 



There is no recognized standard by which authors guage gene- 

 ric characters, and the greatest want of uniformity in custom 

 unfortunatelv prevails among genus makers. Structure is of alto- 

 gether more consequence than form, color, or dermal ornamenta- 

 tion ; but the generic value of any character will depend on its 

 invariability and rarity within the Family or Order. Wing- 

 venation, on which systematists most rely in classifying moths, 

 may be, and often is, of less value, on account of its variability, 

 than special body covering. The abdominal spines and the gen- 

 eral hairiness of Paleacrita are such vei-y rare characteristics in 

 the Family and even in the Order to which it belongs, that they 

 acquire an importance which they otherwise would not possess. 



