1. Gunther, A., Ann. Mag. Hat. Hist., 5, Ser. 3 (1860).
2. Gudger, E. W., Ann. Mag. Nat. Hist., 2, Ser. 9 (1918).
3. Norman, J. R., Nature, 143, 52 (1939).
4. Bonnell, B., Nature, 191, 403 (1961).
5. Storms, R., Ann. Mag. Nat. Hist., 2, Ser. 6 (1888).