Published in Proc. Calif. Acad. Sci. (4), in 1958, in volume 29, pages 257-265

Published in Proc. Calif. Acad. Sci. (4), in 1958, in volume 29, pages 267-272

Published in Proc. Calif. Acad. Sci. (4), in 1958, in volume 29, pages 273-313