The sphinxes and other animals that guarded tombs in the archaic period tend to convey immobility and permanence. In classical lions, however, movement is implicit: they appear to be watching, lying in wait, preparing to spring. With the suggested potential for imminent motion comes a concomitant emphasis on the animals' strength and litheness. The lion here looks to his left, as we anticipate the movement of his muscles and of the tufts of his mane when he takes off after his prey.
Richter, Gisela M. A. 1910. "Department of Classical Art: The Accessions of 1909." Bulletin of the Metropolitan Museum of Art, 5 (2): p. 40.
Richter, Gisela M. A. 1954. Catalogue of Greek Sculptures. no. 145, p. 82, pls. 105c-d, 106, Cambridge, Mass.: Harvard University Press.
Lazzarini, Lorenzo and Dr. Clemente Marconi. 2014. "A New Analysis of Major Greek Sculptures in the Metropolitan Museum: Petrological and Stylistic." Metropolitan Museum Journal, 49: pp. 120-1, 138-9, fig. 8, Appendix.