After the appearance of temporal fenestrae, selection for the classical factors is quite acceptable to explain the further development of fenestration. The continued enlargement of the temporal fenestrae in the pelycosaur-therapsid lineage undoubtedly was correlated with the advantages accrued from securing greater space to allow increased lateral expansion of contracting mandibular adductors. Similarly, weight in absolute terms can reasonably be suggested to explain the dramatic fenestration in the skeletons of many large dinosaurs. Literature Cited ADAMS, L. A. 1919. Memoir on the phylogeny of the jaw muscles in recent and fossil vertebrates. Annals N. Y. Acad. Sci., 28:51-166, 8 pls. ESTES, R. 1961. Cranial anatomy of the cynodont reptile _Thrinaxodon liorhinus_. Bull. Mus. Comp. Zool., 125(6):165-180, 4 figs., 2 pls. HOTTON, N. 1960. The chorda tympani and middle ear as guides to origin and development of reptiles. Evolution, 14(2):194-211, 4 figs. OLSON, E. C. 1961. Jaw mechanisms: rhipidistians, amphibians, reptiles. Am. Zoologist, 1(2):205-215, 7 figs. ROMER, A. S. 1928. Vertebrate faunal horizons in the Texas Permo-Carboniferous redbeds. Univ. Texas Bull., 2801:67-108, 7 figs. 1956. Osteology of the reptiles. Univ. Chicago Press, xxii + 772 pp., 248 figs. ROMER, A. S. and PRICE, L. I. 1940. Review of the Pelycosauria. Geol. Soc. Amer. Special Papers, No. 28, x + 538 pp., 71 figs., 46 pls. WATSON, D. M. S. 1948. _Dicynodon_ and its allies. Proc. Zool. Soc. London, 118:823-877, 20 figs., 1 pl. 1954. On _Bolosaurus_ and the origin and classification of reptiles. Bull. Mus. Comp. Zool., 111(9):200-449, 37 figs. _Transmitted December 5, 1963._ 30-1522 End of the Project Gutenberg EBook of The Adductor Muscles of the Jaw In Some Primitive Reptiles, by Richard C. Fox