2o6 The American Geologist. April, 1903. 
in connection with the definitive body. The analogy with the 
crinoid stem extends further also on genetic and histologic 
relations. In both the tubes are wide in the older forms and 
become more and more constricted in the later by expulsion of 
the intestines. ... In older crinoids with wide stems, the 
cord must have enclosed a considerable portion of the somatic 
cavity and have been in connection with the root, but in later 
forms this whole portion has been reduced to a fibrous tissue 
that no longer shows distinct processes of differentiation and 
therefore has been differently interpreted (Comp. Jaekel in 
Stammesgeschichte der Pelmatozoen, I, 1890, p. 77-79). 
"Very similar are the special conditions of the formation 
of the siphuncle. In the older forms it is frequently very wide ; 
with the later it is mostly narrow, and it becomes finally rudi- 
mentary when the shell was never attached. 
"The remarkable expansions of the isiphonal tube of or- 
thoceratites, which hitherto could not be accounted for, would 
now appear to find their explanation in the supposition, that a 
constriction of the air-chamber would increase the weight and 
thereby the ^stability of the shell upon the bottom. The siphon- 
al calcareous secretions would also have to be regarded as for 
the same purpose." 
Our ispace does not permit us to reproduce here the auth- 
or's explanatory remarks of his theses 5 and 6, but we con- 
sider the views presented on behalf of thesis 7 so suggestive 
that we cite them in part : 
"To thesis 7, referring to the contraction of the ostium of 
various nautiloids, as in Gomphoceras, Phragmoceras. Tetra- 
meroceras and Hexameroceras, I desire to add the following 
explanation : To begin with, it cannot be doubted that accord- 
ing to the current view the single median sinus [of the aper- 
ture] corresponds to the position of the funnel, and that the 
paired symmetric sinuses of the infolded upper shell-margin 
embraced the arm bases and that hence the mouth lay between 
them. These are the only fossil ammonoids which give some 
clew to the form of the cephalosoma and specially to the po- 
sition of the arms on the head, but which still have been little 
considered in this connection. The interest in these ostial 
forms is still hightened by the fact that of nearly related forms, 
such as Tetrameroceras, Hexameroceras, Octomeroceras, the 
