232 Royal Society .— 



this is at length lost by the constriction passing on, and the round 

 form is regained in about a minute. This wave reappears and 

 travels forward again without any distinct period of rest, and I have 

 seen these movements continue for forty-five minutes, though towards 

 the latter part of this period they are less distinct and more limited 

 in extent. The germinal mass has itself during these contractions, 

 which strongly resemble the peristaltic movements of the intestine, 

 undergone changes in form, and has increased in bulk and distinctness. 

 These movements are unaffected by weak galvanic currents. 



During the passage forward of each wave of contraction there is 

 an oscillation of the whole mass of the yelk, so that its germinal 

 pole passes once to the right and ouce to the left of the micropyle, 

 to which it at first corresponded. The plane of this oscillation may 

 be vertical, horizontal, or inclined, but always cuts the micropyle ; 

 it begins and ceases with the contractions ali'eady mentioned, and 

 would seem to be a mechanical result of them. 



For some time before cleavage begins, the only changes of form are 

 the appearance of wave-like elevations and depressions along the 

 under surface of the germinal mass, and its alternate concentration 

 and diffusion. Cleavage begins in about two hours after impreg- 

 nation ; no embryonic cell was observed before it began, nor in any 

 of the cleavage masses. 



The inner membrane is folded in during cleavage ; it is easily 

 seen thrown into folds at the cleft, and for this reason I do not 

 consider it a yelk-membrane, which term would be better applied to 

 the chorion. 



December 21, 1854. — The Lord Wrotteslej% President, in the Chair. 



"Remarks on the Anatomy of the MacgilUvrayia pelayica and 

 Cheletropis Huxleyi (Forbes) ; suggesting the establishment of a new 

 genus of Gasteropoda." By John D, Macdonald, R.N., Assistant- 

 Surgeon H.M.S. Herald. 



Having examined the anatomy of the MacgilUvrayia pelagica and 

 several smaller species of pelagic Gasteropoda, not exhibiting the 

 least similarity in the character of their shells, the author found that 

 they all presented a very close relationship to each other in the type 

 of their respiratory organs, and in other points of structure of less 

 importance. 



The gills in every instance seemed to be fixed to the body of the 

 animal immediately behind the head, and did not appear to be ap- 

 pended to the mantle, as in the Pectinibranchiata properly so called. 

 They were also invariably four in number, and arranged in a cruci- 

 form manner round a central point. They were free in the rest of their 

 extent, elongated and flattened in form, with a pointed extremity, 

 and fringed with long flowing cilia, set in a frilled border. They 

 were, moreover, furnished with muscular fibres, both transverse and 

 longitudinal, and exhibited great mobility when protruded, but lay 

 side by side in the last whorl of the shell when retracted. 



The nuditory capsules, each containing a spherical otolithe, were 



