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of Psiloceras and one of Anomaloceras and in pathologic indi- 

 viduals, always in or approximate to the mesal plane. There is, 

 however, a fact to be noted. In the Endoceratidae and Actino- 

 ceratidae it is always in direct connection with the cicatrix. In 

 other forms not having an endosiphuncle* this connection is 

 not strictly maintained, and while it is often situated over the area 

 of the cicatrix, it may be, as in Eutrophoceras Dekayi, near the shell 

 but not over the cicatrix, or as in Hercoceras, Fig. 13, PI. viii, at 

 some distance from the apex. There is upon the whole, however, 

 a distinct tendency towards location in the mesal plane and centren 

 or ventrad of the centre, those having a subdorsan siphuncle like 

 some species of Eutrephoceras and Aturia being exceptional. In 

 most forms, even those having siphuncle subdorsan in the second 

 septum, it is nearer the venter in the first septum ; marked ex- 

 amples of this are the Eutrephoceras Dekayi of the Cretacic, Fig. 

 4, PL xiii, and several species of Schroederoceras, or else it tends 

 towards the centre, as in Trocholites canadensis, Figs. 39 and 40, 

 PL v. It is also to be observed that in the adults of most forms 

 of Nautiloids the siphuncle is centren or ventrad of the centre, that 

 is on the same side with the hyponome. This tendency is more 

 general among arcuate and coiled Nautiloidea than among straight 

 forms, which as a rule have the siphuncle centren, f and finally in 

 the Ammonoidea the subventran position is universal. 



Whatever may be the cause of the general tendency of each 

 genetic series to evolve along parallel lines of modification so far as 

 the tendency towards coiling is concerned, it is obvious that it is 

 not dependent upon time, climate or any special differences of 

 structure. The bending takes place in every series even in the 

 Piloceras with a huge siphuncle filled with calcareous matter and 

 there is no positive proof that they may not have had coiled forms 

 which belong to the same genetic series although none have been 

 found. Arcuate and coiled shells are also found in every period, 

 and under every condition of climate so far as geographic distribu- 

 tion is concerned. 



It has been assumed in the Introduction that differences of habit 

 could be used to account for these general tendencies producing 



* See Introduction, p. 412. 



tThe exceptions to this rule are very interesting. They include the radical type Di- 

 phragmoceras, the Endoceratidae and the remarkable genus Bathmoceras. All of these 

 have the siphuncle in most examples ventrad of the centre and in many of them it is 

 subventran. 



