In a former paper (Trans. Roy. Soc. Edin., vol. xxxiii. part ii. p. 465) the ellipsoid under consideration was supposed to have its three axes different from one another in length. I consider now the more simpler case of an Ellipsoid of Revolution. The same notations as in the former paper shall be made use of, and for the establishment of some of the results which form the basis of the present paper, reference must be made to the former.

Excepting the few species noticed in the Report on the Ostracoda of the “Challenger” Expedition, scarcely anything, so far as I know, has been published respecting the Ostracoda of the South Sea Islands. Prof. G. M. Thomson has indeed published in the Transactions of the New Zealand Institute (1878), a paper on Crustacea, which includes a few marine and fresh-water Ostracoda of New Zealand; and the Rev. R. L. King, in the Proceedings of the Royal Society of Van Diemen's Land (1855), described numerous species of Entomostraca, amongst which were several fresh-water, but no marine, Ostracoda. Dr Baird also published a species of Cypridina from New Zealand. I have myself contributed to the Proceedings of the Zoological Society of London (1886) a paper on Entomostraca collected in South Australia, chiefly by Professor Ralph Tate of Adelaide, including a considerable number of fresh-water Ostracoda; and in a French publication (Les Fonds de la Mer), edited by the Marquis de Folin, there are likewise, by myself, descriptions of a few species taken at Nouméa, New Caledonia. There are also, in a paper of mine published in the Transactions of the Zoological Society (1865), notes of a few Australian marine species. This, I think, represents the sum of our present knowledge respecting the Ostracoda of these regions.

Among most of the geologists who had of late years been engaged in investigating the structure of the North-West Highlands, and especially among those who did not concur in Murchison's explanation of the phenomena exhibited there, it was a growing belief that great overthrusts had been largely instrumental in producing the remarkable stratigraphical relations of the rock masses of that region. After a most careful detailed examination of the ground by the Geological Survey, the existence and importance of such thrusts was not only placed beyond a doubt, but a variety of additional remarkable structures were discovered, which open up new fields of investigation to the physical geologist.

The phosphines which have been obtained as yet are not very numerous, and with one or two exceptions their properties have not been exhaustively studied, a fact which is no doubt largely due to the difficulties encountered in preparing them in any quantity.

There are consequently a great many points in their history which require examination, and the object we had in view in the present investigation was chiefly to extend our knowledge of this interesting group of substances.

Some time since I received from Mr W. Smith of Ashburton, New Zealand, a single example of a worm belonging to the little known genus Phreoryctes, and published in the Annals and Magazine of Natural History for June 1888, a short account of the reproductive organs of this worm, for which I proposed the name of Phreoryctes Smithii.

More recently (November 1888) I have received, through the kindness of the same gentleman, a large number of specimens of the same species; the study of these enables me to offer to this Society a more complete account of the structure of Phreoryctes, which is at present very imperfectly known. It will be seen, however, that the present paper, though more extended, and dealing with organs to which little or no reference was made in my former paper, contains no important correction of facts stated in that paper. I have also appended (p. 638) a discussion of the systematic position and affinities of Phreoryctes.

Comparative anatomists have long desired detailed information on the placentation of the Sirenia. It is true that in 1878 Dr Paul Harting presented to the University of Utrecht a graduation thesis, in which he described the fœtal membranes and fœtus of a Dugong, which had been acquired a short time before for the Zoological Museum of that University. The specimen had been preserved for many years in spirit of wine, and had apparently been collected by a surgeon to a merchant ship. The fœtus was 27·8 centimetres (11 inches) long. The entire chorion, with the exception of the two poles and their immediate vicinity, was covered by densely packed, short and but little branched villi, and Dr Harting came to the conclusion that the placenta of the Dugong was diffuse and non-deciduate.

1. To complete the census of knots of any given order, that is, minimum number of crossings, it is necessary to include not only those in which the crossings are taken alternately over and under (Alternate ±), but also the Non-Alternate ±, those in which two or more consecutive crossings are alike over or alike under. Professor Tait has figured the forms of the alternate ± knots, of orders three to nine inclusive, on Pl. XLIV. vol. xxxii., Trans. Roy. Soc. Edin.; the object of this paper is to describe the non-alternate knots of these orders.