Edinger, op. cit. p. 678.
T. Jeffery Parker, op. cit. p. 55.
Archiv f. mikr. Anat. xiii. 1876.
Krukenberg, quoted by Miss Alcock, Journ. Anat. and Phys. xiii. (N.S.), 1899, p. 613.
Stannius, Handb. d. Zool., Berlin, 1854, ii. p. 201; Owen, op. cit. p. 425.