the Genus Eschar a. 175 



traverse the pores or spiracles of the surface, and produce the 

 incrustation externally, and not internally, as in shells. In some 

 polypidoms these pores are placed on the anterior surface, 

 whilst in others, as in the Retipora, they are on the posterior 

 side. 



" Another observation, which, so far as I know, has not pre- 

 viously been made, is the discovery of pores of communication 

 between the cellules, and which pores establish the commonalty 

 of life in all the porous polypidoms, and at the same time pro* 

 mote the propagation by gemmation, or the emission of the 

 gemmules,* which develope themselves at the free edge and ex- 

 tremity of the polypidom, and promote its increase ; whilst the 

 eggs which are destined for the reproduction of the species in 

 another part are found in particular capsules above the opening 

 of the cellule. These communications, which I have discovered 

 in all living polypidoms, where they are sometimes formed by 

 a cribriform plate, may be observed in the fossil polypidoms 

 which I have represented when the crustaceous matter has been 

 worn down on either side ; and I have been solicitous to re- 

 present them in both aspects. 



" It follows from this fact, that we cannot admit an isolated 

 life to the Eschara, or the other cellular polypi, at any period 

 of their development. And it is especially on this account that 

 I wish to mark a date, by presenting these plates to the Aca- 

 demy." 



The Magnetic Variation near Edinburgh, and in Berwickshire. 

 By Captain MILNE, R. N., F. R. S. E. Communicated by 

 the Author. 



THE amount of the magnetic variation at different stations, 

 will, after an interval of several years, necessarily be a subject 

 of scientific interest, not only from its connexion with naviga- 

 tion, but more particularly from its being illustrative of the 

 still unknown laws of magnetism. The following observations 

 have been made, and are now published, with the view of af- 



* These corpuscules differ from true ova and seeds, inasmuch as the sub- 

 stance of which the new being is formed is not inclosed in a special envelope, 

 and its formation is owing to the active substance of the reproductive cor- 

 puscule. Tiedemann's Comp. Phy. 42. 



