1. Grave, C., and Nicoll, P. A., Carnegie Inst. Wash. Pub. No. 517, Papers Tortugas Lab., 32, 1 (1939).
2. Grave, C., J. Morph., 75, 173 (1944).
3. Glaser, O., and Anslow, Gladys A., J. Exp. Zool., 111, 117 (1949).
4. Berrill, N. J., The Origin of Vertebrates, 125 (London, 1955).
5. Bell, L. G. E., Nature, 175, 1001 (1955).