254; THE MAEQUIS DE FOLIK ON THE CiECIDiE. 



Pig. 5. View of a nearly completed specimen ; the regenerated anterior 

 portion, consisting of the head and the oesophageal region of the di- 

 gestive chamber, is recognized by its pallor : b, cephalic fissure. Mag- 

 nified under a lens. 



Observations on the Septum of the CaecidsB ; and some remarks 

 on the subject of the suppression of the Genera Brochina 

 and Strebloceras or Phleboceras. By the Marquis Li:oPOLD de 

 FoLiN. Communicated by W. Bated, M.D., E.E.S., F.L.S., 



«&c. 



(Plate VIII.) 



[Read June 18, 1868.] 



The importance of the septum, or that part of the shell, in the 

 Csecid£e, whose function is to close the tube when a portion of it 

 has become useless and been abandoned, has been considered 

 very great. But whilst recognizing the value, as a diagnostic 

 specific character, of the restored apex, I am, nevertheless, of 

 opinion that it should be employed with circumspection in the 

 case of specimens distinguished from each other only by slight 

 differences in this respect. I have observed such imperceptible 

 gradations between two septa apparently most widely distinct, 

 tliat, it appears to me, but very doubtful reliance can be placed 

 on the character presented by that part of the shell, and that it is 

 always necessary to guard against our being deceived by certain 

 abnormities in form which occasionally affect its value. 



It is possible that by paying some attention to the mode in which 

 the obliteration is effected (by which is meant the mode of formation 

 of the septum), we may obtain some aid in support of this opinion, 

 or which may afford the means of explaining it. 



It appears evident that the production of an apex to the new 

 shell, as yet incompletely formed, commences by a circular suture 

 on the inner wall of the tube. The plane in which this circum- 

 ferential suture lies will be termed the " plane of obliteration ; " 

 and we may remark at starting that a section which would sepa- 

 rate the older from the new shell would lie most frequently in 

 this plane. We will now speak of the desertion of the shell when 

 it has become insufficient for its inhabitant. 



It is clear that in attempting to account for the manner in 

 which the mollusk proceeds in tlie construction of this septum 



