My findings and interpretations do not support previous suggestions that the thoracic artery has undergone a mediad migration, and that the various sites of attachment of that vessel may come to represent various levels of evolution. The primitive site of attachment of the vessel is unknown, and it seems to me that it has not been sufficiently demonstrated that the vessel has undergone any "migration." [Illustration: FIG. 1. _Progne subis._ Lateral view of left half of thorax. _M. pectoralis thoracica_ (area of insertion indicated by dotted line) has been removed. Muscles not described in text are not shown. (x 1.5.)] [Illustration: FIG. 2. _Progne subis._ Lateral view of left half of thorax. Same view as shown in Fig. 1, but with _Mm. supracoracoideus_, _coracobrachialis posterior_, and _intercostales externus_ removed. (x 1.5.)] [Illustration: FIG. 3. _Progne subis._ Medial view of left half of thorax. Not all muscles shown. See Fig. 4 for identification of arteries. (x 1.5.)] [Illustration: FIG. 4. _Progne subis._ Lateral view of left half of thorax. (x 1.5.) (Applies also to Fig. 3.) 1. Subclavian artery. 2. Coracoid artery. 3. Thoracic artery. 4. (Unnamed.) Supplies _M. coracobrachialis posterior_. 5. (Unnamed.) Supplies _M. subcoracoideus_. 6. (Unnamed.) Supplies _M. sternocoracoideus_ and sternum. 7. (Unnamed.) Supplies _M. subcoracoideus_. 8. (Unnamed.) Supplies _M. sternocoracoideus_, _M. coracobrachialis posterior_, and sternum. 9. Pectoral artery. 10. Axilliary artery. 11. (Unnamed.) Supplies _M. coracobrachialis posterior_. 12. (Unnamed.) Supplies _M. supracoracoideus_. 13. (Unnamed.) Supplies _M. costi-sternalis anterior_. 14. Carotid artery.] [Illustration: FIG. 5. _Scardafella inca._ Lateral view of left half of thorax. _M. pectoralis thoracica_ (area of insertion indicated by dotted line) has been removed. Muscles not described in text are not shown. See legend for Fig. 7 for identification of arteries. (x 1.)] [Illustration: FIG. 6. _Scardafella inca._ Lateral view of left half of thorax. See legend for Fig. 7 for identification of arteries. (x 1.)] [Illustration: FIG. 7. _Scardafella inca._ Medial view of left half of thorax. (x 1.) KEY (Applies also to Figs. 5 and 6.) Numerals not used are those used for _Progne subis_ for which no homologous artery occurs in _Scardafella inca_.