96 BULLETIN OF THE 



of the coccum nearer the centre accords with the situation of the first 

 septum, and its parallelism to the area of the cicatrix, but it is still, as in 

 the recent Nautilus, considerably nearer to the ventral than to the dorsal 

 side. Owing, however, to the position of the first septum with relation 

 to the angle or apex made by the bending of the external shell around 

 the ventral edge of the first septum, the coecum is not so near the so- 

 called apex of the whorl as in Nautilus Pompilius. In other words, it 

 lies nearer the centre of the actual apex, the centre of the cicatrix, and 

 farther away from the apparent apex, or angular termination of the 

 outline, than in Nautilus Pompilius. The peculiar abrupt curves which 

 it subsequently makes are due partly to the abruptness of the ventral 

 side of the coccum, and partly to the angle at which the first septum lies 

 with relation to the second. All of these characteristics are thus shown 

 to be dependent upon the altered or accelerated development of the 

 septa in the young of Nautilus Pompilius, which causes the first septum 

 to assume a position and other characteristics similar to those of the 

 third septum of Nautilus atratus and lineatus. Another fact in the 

 same direction is the ventral position of the coecum in the Jurassic and 

 existing species, as contrasted with the central siphon of the young of 

 the Silurian and Carboniferous Nautili. The extreme variability of 

 position of this organ among the adult forms of the Silurian of course 

 renders this characteristic somewhat doubtful, but it is a curious fact that 

 we should find it so strictly accordant or correlative with the other char- 

 acteristics already described. It must evidently be added to our other 

 list of characteristics, which though at first variable, become in course of 

 time fixed and invariable, through the action of the law of acceleration. 

 The siphon of Goniatites differs in a remarkable manner from that of 

 Nautilus. It has a long conical termination which penetrates the first 

 septum, and lies so close to the abdomen as to form a very decided fis- 

 sure, probably due, however, as previously explained, to the removal of 

 the shell. Otherwise the siphonal coecum would be open on the ab- 

 dominal side, which it does not seem to be when seen from the side, and 

 covered by the shell. The coecum above is flask-shaped, the neck of 

 the flask lying between first and second septa. The first septum forms 

 the round bottom of the flask and the closed conical prolongation ; the 

 second septum, the neck' and part of the body of the upper portion. 

 The neck, however, continues to decrease in size until it reaches the 

 third septum. The siphonal funnel is apparent even in the first septum, 



