762 THE: PALEONTOLOGY OF MINNESOTA. 
{Introductory . 
In both of these subdivisions the shell may be straight, arcuate or spirally 
enrolled; but in the Nautiloidea the sutwres, or lines of junction of the septa with 
the walls of the shell or conch, are, as a rule, simple, without abrupt curvature and 
very seldom with sharp angles, while in the Ammonoidea these sutures are usually 
highly angulated or zigzagged; in the nautiloids the sipho is very variable in 
position, may be small and cylindrical, but is often large and its walls much thick- 
ened, while in the ammonoids the sipho is always cylindrical, always marginal in 
position and without a thickening of the walls. 
In the faunas of the Lower Silurian no representatives of the dibranchiates or of 
the ammonoid tetrabranchiates are known. We have, therefore, to deal in this 
chapter only with the nautiloid tetrabranchiates. 
As an illustration of the general structure of these bodies and to show the 
relation of the animal to the various parts of the shell, we have here inserted a 
drawing of Nautilus pompilius, reproduced from the well known figure by Richard 
Fig. 1.—Nautilus pompilius. 
a. Mantle. n. Hood. 
b. Its dorsal fold. o. Exterior digitations. 
e. Nidamental gland. p. Tentacles. 
g. Shell muscle. s. Eye. 
i. Sipho. a. Septa. 
k. Funnel. z. Body chamber. 
