587 
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 Endoceratide and Actino- 
ceratide it is always in direct connection with the cicatrix. In 
othet 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 Dekayt, near the shell 
but not over the cicatrix, or as in Hercoceras, Fig. 13, Pl. villi, 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 Zrocholites 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,+ 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. 
+The exceptions to this rule are very interesting. They include the radical type Di- 
phragmoceras, the Endoceratide and the remarkable genus Bathmoceras. All of these 
haye the siphuncle in most examples yentrad of the centre and in many of them it is 
subyentran. 
