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riably towards the dorsum, but that this is the general tendency of 

 arcuate forms is obvious. 



When it comes to the evolution of coiled forms the problem is 

 different. Among these last, including also the loosely coiled or 

 gyroceran, there is so far as known no exception to the rule that all 

 such shells are bent dorsally from the earliest substages of the 

 conch. 



I have assumed in other papers that coiling among Gasteropoda 

 could be accounted for by the unequal growth caused by the weight 

 of the shell when carried above the foot and the facts appear to 

 justify this conclusion in so far as that class is concerned. The 

 presence of the hyponome on the ventral side in Cephalopoda 

 would of itself account for the tipping of the shell towards the 

 opposite or dorsal side both when the animal was crawling and 

 swimming. This would leave the ventral edge of the mantle free 

 to deposit calcareous matter undisturbed by pressure, whereas the 

 dorsal edge would be more subject to disturbance and to shocks 

 from compression which might interfere with the work of excretion. 

 It is reasonable to suggest such a mechanical explanation both for 

 the general tendency to bending and coiling and also for the dorsal 

 direction. 



If it were possible to account for the exceptions observed, as in 

 the tendency of Phragmoceras towards the venter, by means of 

 exceptional habits or structures, this suggestion would have more 

 force, but unfortunately this cannot be done, at least at present. It 

 is obvious, however, that there is some directive cause which acts 

 upon every genetic series in greater or less proportion, giving to 

 each evolving series the same tendency to produce in succession 

 the straight, arcuate,* and then the coiled forms in different de- 

 grees of intensity and that most of these have coiled in a dorsal 

 direction away from the hyponome or organ of locomotion. 



The position of the siphuncle with reference to the mode of 

 coiling need not be discussed, since it obviously has no general 

 relations, except that it is always, except in turbinate forms, Tro- 

 choceras, Turrilites, etc., and in abnormal forms, like some species 



* Even in large and some small species and specimens of Baculites there is an arcuate 

 tendency. D'Orbigny figures this in B. incurvatus, Terr. Cretac, PI. cxxxix, and Mr. 

 Stanton has put together a very large specimen of Baculites in Nat. Mus. having a curve 

 like Cyrtoceras. This is some five feet long, straight or nearly so in the younger part 

 and arcuate in the older stage. 



This specimen is from the Ripley Formation, Texas. 



