BY H. LEIGHTON KESTEVEN. 449 



if this be not admitted, it becomes impossible to define the 

 embryonic stages. 



Taking as my base the genus Lotoriuia, in examining the 

 second type of transition I am presented with the following- 

 facts. The true protoconch is horny and contains practicall}^ no 

 calcareous matter. During the formation of at least part of this, 

 the organism is a free-swimming veliger; the same applies to 

 Gyrineitru australasia, Perry. The next stage I am acquainted 

 with is that in which a little less than half a whorl of neanic 

 structure has been added. The protoconch has now deposited 

 within it distinct traces of the pseudoprotoconch in the form of 

 an extremely thin layer of calcareous lining, the neanic structure 

 being much thicker and exhibiting the adult sculpture in minia- 

 ture. The mollusc itself is sedentar}^ and has lost all traces of 

 the velum. The abrupt transition from one structure to the 

 other may be explained by the sudden functional activity of the 

 secondary shell-secreting area. It is probable that during the 

 nepionic stage, which was slightly protracted, there was a com- 

 plete cessation of shell-growth, and that the primitive gland had 

 ceased its function before the secondary shell was initiated. 



As an example of the third class of transition, perhaps the 

 rarest and most interesting, Miirex dfmndata, Perry, exhibits the 

 following characters : — A stout calcareous protoconch longi- 

 tudinally sculptured, followed by a prominent varix, the succeed- 

 ing neanic structure exhibiting, in miniature, all the adult char- 

 acters. The sculpture of the protoconch is such as to prove 

 conclusively that it was not cast inside a horny mould and is 

 therefore a true protoconch. The embryo thus had the ahilitij to 

 secrete a calcareous sheU. It seems reasonable to suppose that 

 during the nepionic stage (during which there must generally 

 be a longer or shorter pause in the growth of the mollusc) the 

 secretion of shell was carried on by the free edge of the mantle. 

 Granted this, the varix maybe looked upon as the conchylaceous 

 record of the nepionic stage. 



Conclusions arrived at. — The perfection of internal organs 

 during the veliger stage, postulated above for Triphora, has been 

 28 



