Note 23 — Chapter III · SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA.
Reading edition
See E. Ray Lankester, Quart. J. Micr. Sci. vol. XXIX. n. s. 1889, p. 365. W.B. Benham, Ibid. vol. XXXV. n. s. 1893, p. 97. J.W. Kirkaldy, Ibid. vol. XXXVII. n. s. 1895, p. 303. The last-named writer divides the genus into three subgenera.