138 



The part of the Cornu ammonis which next particularly demands our 

 attention, is the siphuncle ; and which has hitherto been so rarely made 

 out, as to have led many to doubt whether it existed or not. Its ex- 

 istence being, however, proved in several specimens, analogy leads us 

 to conclude that it really exists in all ; and that it served the same pur- 

 poses as the siphuncle in the Nautilus, the enabling of the animal to 

 regulate/ its ascent and descent in the water. 



The sporidylolite, Plate IX. Fig. 3, strongly proves the existence of 

 the siphuncle, by the deeply-sunk circular cavity which exists on the 

 outer side of this fossil body, where, doubtlessly, the siphuncle had 

 passed. In the very rare specimen, Plate IX. Fig. 4, which formed a 

 part of the Leverian collection, the siphuncle itself is seen, in situ. 



The situation of the siphuncle is not always the same, in every spe- 

 cies: in some, it runs along the back of the shell, as in the specimen 

 Fig. 3 ; in others, it runs along the inner part of the whirl, as in the 

 specimen represented Fig. 4 ; and, unless I am very much deceived, a 

 siphuncle runs along both the outer and inner side of the whirls of the 

 specimen, Fig. 5. 



The exact figure of the siphunculus is not, I believe, yet known. I 

 have destroyed many specimens, with the hope of obtaining information 

 in this respect ; but without decided success. In general, it appears to be 

 formed by a uniform cylindrical tube, regularly increasing in size as it 

 proceeds from the central to the outer whirls. 1 think, however, that 

 in some parts of the specimen, Plate IX. Fig. 5, the siphuncle seems to 

 be a little contracted in those parts where the septa unite with the 

 outer shell ; but that, in other parts, the septa appear to be formed of 

 two shelly plates, which, separating as they approach the outer shell, 

 seem to admit a duplicature of the siphuncle to dip in between them. 

 Some of this appearance may however be a deceptio visits, proceeding in 

 some measure from the direction in which the parts are divided. 



Bertrand, Diction, des Fossiles, p. 175, and others, who have been 

 aware of the existence of this siphuncle, have yet supposed that this 



