LEE BARKER WALTON 13 



while a homologous part can generally be demonstrated 

 throughout the other orders. Furthermore, it is to be 

 noted that in the typical form this is joined to the dorsal 

 margin of the episternum, while the wing is articulated 

 with the dorsal margin of the epimeron and not, as hither- 

 to accepted, with the episternum.^ 



In connection with the facts noted above, certain evi- 

 dence is available, based on embryology as well as com- 

 parative anatomy, which adds weight to the inference 

 that these two pieces are rudimentary- wings and that the 

 thorax of the Hexapoda is composed of six somites which 

 in the typical form bear the fundaments of six pairs of 

 tuings. 



The development of the mesothoracic pterygodum^ 

 ( = tegula, parapteron, etc.) is of extreme interest in this 

 connection and furnishes interesting evidence toward 

 establishing the hypothesis I have put forward regarding 

 its relation to the wing. This, however, is only one of a 

 large number of facts which corroborate such a view and 

 which appear explainable on no other basis. To one of 

 these I have already called attention (Walton, 1900), 

 notably the formation of the coxa in Chilopoda and Hex- 

 apoda from two fused pieces to which I have applied the 

 name "coxa genuina" and "meron." 



I have adopted the name "pterygodum"^ for the present 

 in preference to others which have been suggested for the 

 mesothoracic piece (tegula, parapteron, squamula, etc.), 

 since it has priority over terms otherwise acceptable, and 

 according to our present knowledge better indicates the 

 function of the part. The term "parapteron," which 

 Comstock and Needham ('98), following Newport ('39), 

 have used in reference to the mesothoracic pterygodum, 



^The anterior margin of the wing is generally specialized at the base, so that it 

 partially extends over the dorsal portion of the episternum, while an articulative 

 process of the pterygodum may be received into a corresponding notch of wing. 

 This condition, however, is secondary. 



'There appears to be no evidence for regarding these as vestigial, since it is im- 

 probable that forms with six pairs of wings ever existed. The term "rudimentary," 

 as generally used, does not seem inappropriate, although the word "fundament" is 

 more concise. 



Owing to its greater size, this can be more conveniently studied than the cor- 

 responding piece on the metathorax. 



'*<Cgr. TrTepv'YUidr]s<C TTTepvyoeiSrjs = irripvyos [wing] -j- eldos [form]. 



