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 sutures, 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 

 tne 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 



a. Mantle. 



b. Its dorsal fold. 



e. Nidauental gland. 



Shell muscle. 

 i. Sipbo. 

 k. Funnel. 



:/. 



Fig. 1. Nautilus pompilius. 



n. Hood. 



o. Exterior digltations. 



p. Tentacles. 



. Eye. 



x. Septa. 



z. Body chamber. 



