I gratefully acknowledge many valuable suggestions in my research and the preparation of this manuscript from Professors Theodore H. Eaton, A. Byron Leonard, Richard F. Johnston, Robert M. Mengel, and E. Raymond Hall. Mr. Abbot S. Gaunt and Miss Sandra Lovett assisted in collecting specimens. Final drafts of the illustrations were prepared by Mr. Thomas Swearingen. MYOLOGY AND ANGIOLOGY: HIRUNDINIDAE Figs. 1, 2, 3, and 4 illustrate the following muscles and arteries described for _Progne subis_. Myology ~_M. pectoralis thoracica_~, Fig. 1. The origin is from slightly less than the posterior half of the sternum, from the ventral half of the keel, almost the entire length of the posterolateral surface of the clavicle and adjacent portion of the sterno-coraco-clavicular membrane, and tendinously from the ventral thoracic ribs. This massive muscle covers the entire ventral surface of the thorax and converges to insert on the ventral side of the humerus on the pectoral surface. ~_M. supracoracoideus_~, Fig. 1. The origin is from the dorsal portion of the keel and medial portion of the sternum, and is bordered ventrally by the origin of M. pectoralis thoracica, and laterally by _M. coracobrachialis posterior_. The origin is also from the manubrium and the anterolateral portion of the proximal half of the coracoid and to a slight extent from the sterno-coraco-clavicular membrane adjacent to the manubrium. This large pinnate muscle converges, passes through the foramen triosseum, and inserts by a tendon on the external tuberosity of the humerus, immediately proximal to the insertion of _M. pectoralis thoracica_. ~_M. coracobrachialis posterior_~, Figs. 1 and 3. The origin is from the dorsolateral half of the coracoid, anterolateral portion of the sternum (where the area of origin is bordered medially by _M. supracoracoideus_, posteriorly by _M. pectoralis thoracica_, and laterally by _M. sternocoracoideus_), and also to a slight extent from the area of attachment of the thoracic ribs to the sternum. The muscle fibers converge along the lateral edge of the coracoid and insert on the median crest of the humerus immediately proximal to the pneumatic foramen. In passing from the origin on the sternum to the insertion on the humerus, the belly of the muscle bridges the angle formed by the costal process of the sternum and the coracoid. ~_M. sternocoracoideus_~, Figs. 2 and 3. The origin is from the entire external surface of the costal process of the sternum, and to a small extent from the extreme proximal ends of the thoracic ribs where they articulate with the costal process. The muscle inserts on a triangular area on the dorsomedial surface of the coracoid. Like _M. coracobrachialis posterior_, this muscle bridges the angle formed by the costal process and the coracoid.