The number of turns in the cochlear spiral and length of the basilar membrane in several mammalian species were compared with the octave range and the high-and low-frequency limits of hearing. Basilar membrane length and the number of spiral turns were not related. Among ground dwelling mammals, the number of turns in the cochlear spiral was more strongly related to octave range than was basilar membrane length. Basilar membrane length was inversely related to the high-and low-frequency limits of hearing. The best estimates of high-and low-frequency limits and octave range were derived from formulas which included both the number of turns in the cochlear spiral and the basilar membrane length as factors. The number of turns in the cochlear spiral was most highly correlated with the difference between the low-frequency limit of hearing and the lowest frequency mechanically analyzed by the traveling-wave envelope, peak-shift property of the basilar membrane [von Békésy, Experiments in Hearing (McGraw-Hill, New York, 1960)]. The coefficient of correlation for the number of spiral turns and the octave difference between the lowest audible frequency and the lowest frequency distributed as a unique point of maximum displacement along the basilar membrane was r = 0.997 (P less than .001) at 60 dB SPL. Mechanisms by which the spiral form of the cochlea may affect the motion of hair cells and the selective response of the tectorial membrane to differences among traveling-wave envelope slopes and peak locations were reviewed. It was proposed that in ground dwelling mammals, the spiral form of the cochlea extends the octave range of hearing and that through mechanisms such as these increases the sensitivity of the cochlea to frequencies below the low-frequency peak-shift limit of the basilar membrane.