ORIGIN OF DIFFERENTIALS. 51 



have been also useful as an air-tight band around the animal, fastening the 

 mantle closely to the shell. The very slight impression made in the inner 

 surface of the living chamber shows that this muscle was not very strong, 

 nor very useful for purposes of prehension, and we are disposed to agree with 

 Dr. Waagen's remark, and even perhaps go a little further. Finely preserved 

 casts of the living chambers of Ammonoids, from Solenhofen and other places, 

 do not afford traces of this annular band, and it seems to have been of a similar 

 nature, but of not so great importance in this order, as the pallial muscles among 

 the Lamellibranchs. The animal of Nautilus was probably held in its shell almost 

 exclusively by pressure, and this band of muscles perhaps served to secure the 

 posterior parts from being disturbed by the movements of the outer pai'ts of 

 the body while the animal was using its hyponome. The supposed muscular 

 band of Oppelia steraspis, figured by Waagen, 1 runs forward on the sides much 

 nearer to the lateral edges of the aperture than in Nautilus. This fact also 

 indicates that the Ammonitinae could not have used the fore parts of the body 

 in the same way as the Nautiloids. 



The convexity of the central zone of the septum is certainly a differential 

 among Ammonoids when compared with Nautiloids, but it is in strict correla- 

 tion with the arising and lengthening of the dorsal, ventral, and lateral lobes, 

 especially the first two, and is therefore concomitant with the increasing com- 

 plication of the sutures, the closer coiling, and the greater involution of the 

 whorl in this order. We have already given the details sustaining this view 

 in our Genera of Fossil Cephalopods, and need only refer here to the cases of 

 Pinnacites (p. 311), in which the septa are double concaves on account of the 

 ridges formed by the large lateral saddles, and the family of the Primordia- 

 lidse (p. 316). "While still in the broad-whorled anarcestian stage, the septa 

 are nautiloidean or concave, but when the deep ventral" and dorsal 2 " lobes and 

 large lateral saddles are formed, the septa become ammonitoid or convex along 

 the median line." It becomes, therefore, necessary to look upon this differential 

 character as also attributable directly to the habits of the animal, and due to the 

 efforts of the Ammonoid to respond to the requirements of its surroundings. 



The persistent ventral position of the siphon is a constant differential among 

 Ammonoids. Even the nostologic series of the Jura and Cretaceous, which 

 yielded so readily to physical changes, becoming uncoiled and departing in 

 many of their characters from the normal ancestral types, still retained the 

 ventral position of the siphon. Although there was considerable lateral varia- 

 tion in the position of this organ in some species, it remained, so far as known, 

 always external or ventral. 



The size of the siphon becomes a matter of considerable importance in this 

 connection, and must be considered as throwing some light on this obscure 

 point in the history of the Cephalopoda. The siphon was a far less important 

 organ among the later than among the earlier Nautiloidea. It was also smaller, 

 as may have been already gathered from what we have said above, 3 in the adult 



1 Op. cit., p. 193, pi. xi. fig. 4. 



2 These two words should have been inserted, but were accidentally omitted. 3 Pages 12-17. 



