1891.] Mr Chree, On thin rotating isotropic disks. 215 



quantities taken between the limits — I and +1 of z is the same 

 as when terms of order l l are neglected. Thus the Maxwell- 

 Grossmann method of solution leads to values for all the radial 

 and transverse strains and stresses which are identical with those 

 which the present solution supplies for mean values taken through- 

 out the thickness of the disk. It also as we have seen when 

 fully worked out supplies the same value for w at the plane 

 surfaces of the disk, and so the same mean value for the longitu- 

 dinal compression throughout the thickness. 



In consequence of the second relation (22) the mutual inclina- 

 tions of all material lines in the disk remain unchanged. Thus 

 what were originally cylindrical surfaces co-axial with the original 

 edges cut orthogonally the surfaces into which have been trans- 

 formed what were originally planes parallel to the faces ; i. e. they 

 become orthogonal to what are practically a series of paraboloids, 

 whose common axis is that about which the rotation takes place. 

 This perhaps will convey the clearest idea of how material lines 

 originally perpendicular to the faces become under rotation con- 

 cave to the axis of the disk. 



May 18, 1891. 



Professor Liveing, Vice-President, in the Chair. 



The following Communications were made to the Society: 



(1) On Parasitic Mollusca. By A. H. Cooke, M.A., King's 

 College. 



[Received July 18, 1891.] 



Various grades of parasitism occur among the Mollusca, from 

 the true parasite, living and nourishing itself on the tissues and 

 secretions of its host, to simple cases of commensalism. Some 

 authors have divided these forms into endo- and ecto-parasites, 

 according as they live inside or outside of their host. Such a 

 division, however, is hardly tenable. Certain forms are indif- 

 ferently endo- and ecto-parasitical, while others are ecto-parasitic 

 in the young form, and become endo-parasitic in the adult. It 

 will be convenient therefore, simply to group the different forms 

 according to the home on which they find a lodgement. 



On Ccelenterata. (a) Sponges. Vulsella and Grenatula almost 

 invariably occur in large masses of irregular shape, boring into 

 sponges, (b) Corals. These form a favourite home of many 

 species, amongst which are several forms of Coralliophila, Rhizo- 

 chilus, Leptoconcha, and Sistrum. The common Magilus, from the 

 Red Sea and Indian Ocean, in the young form is shaped like a 



