25G ,TIIE MAItQUIS DE FOLIN ON THE CMCiDM. 



-less it is Ijy no means extraordinary and can be very easily explained. 

 The fracture was not in the " plane of obliteration," but a little 

 above it ; and an instance of this sort cannot surprise us, seeing 

 that I have had occasion to observe several species in which the 

 mucrouate or dactyliform septum, which is sometimes very much 

 elongated and attenuated, and consequently fragile, appears to 

 require protection. The persistence of a part of the primitive 

 shell, sufficient to conceal the septum, furnishes such a safeguard. 

 In the instance noticed by Mr. Carpenter, the cause of the condi- 

 tion was not the same ; and we can see in it simply an example of 

 accidental fracture of the apex of the shell of the second stage 

 at a time when that of the third stage was still incomplete, or 

 before the proper period for decollation had arrived. But it is not 

 by such an explanation of the circumstance that I shall contest 

 the hypothesis advanced by Mr. Carpenter. I would also 

 remark that, if due attention be paid to the foregoing observa- 

 tions regarding the formation of the septum, they will tend to 

 show that, in order to render a translation of that part possible, 

 we must first admit that it has been preceded by a solution of the 

 primary apex. . In fact we meet with shells of the second stage 

 provided with a septum perfectly united to the walls of the tube, 

 quite complete, and certainly produced by an original process of 

 obliteration. Lastly, with respect to this point, I am in a con- 

 dition to affirm, from specimens in my possession (some in the 

 first stage, others in the first and second united, and some exhi- 

 biting all three stages in the same shell), that all have the spiral 

 apex perfect and entire, and which has not been displaced, never 

 having lost its original position. Moreover, I have never ob- 

 served a septum belonging to the after stages of the shell pre- 

 senting the slightest appearance of spirality. We are thus in a 

 position to conclude that if the mollusk could move without 

 having any support for the extremity of its muscle from the pri- 

 mary apex to the second, it may well be able also to transport itself 

 from the first portion of the second shell to that of the third. 

 This first process towards obliteration may invariably be noticed 

 existing in every shell of the second stage ; and if it were de- 

 stined to be displaced in order to close a subsequent portion, it 

 would not be always found in the same position, that is to say, 

 closing the summit of the second shell. 



Moreover, if we admit the possibility of a translation taking 

 place, it might be asked how such translation is eft'ected. We 



