CHAPTKU IX 



THE LOWER siLt'RlAN CEPHALO 

 I 'ODA OF MINNESOTA. 



ll\ K.IIN M I I.MIKK. 



INTRODUCTORY. 



The Cephalopoda or " head -footed " inollusks are distinguished from the other 

 mnllusran groups by the possession of a circlet of long fleshy tentacles or prehensile 

 organs arranged alxmt the head. 



This group of animals is a very large one and, from its appearance in the early 

 faunas of the globe to the present time, has been represented by species of limitless 

 liv.'rsity in form and structure. Those with which we have to deal in this chapter 

 represent only early and primitive types of structure. 



The two Orders of the Cephalopoda generally recognized are: 



1. Tetrabranchiata; 



2. Dibranchiata; 



terms which imply the possession respectively, of four and two gills. 



The tetrabranchiates are typified by the living Nautilus poinpilius; the 

 dibrauchiates by the Loligo, or squid, Sepia, or cuttle-fish. 



The tetrabranchiates were wonderfully ahmnliiut throughout the Paleozoic an.l 

 Mesozoic periods of the earth's history, hut are to-day almost extinct, while the 

 ililiranchiates are the predominant cephalopoda in existing seas, and their fossil 

 representatives much less numerous and diverse. 



The tetrabranchiates possess shells in which the animal occupies only the 

 outer or forward portion, and the rest of the internal cavity is divided into successive 

 chambers by a series of transverse or oblique plates, called septa. These septa are 

 connected with one another and with the outer or habitation chamber by a fleshy 

 tube or sipho passing though a perforation in each septum. This order is usually 

 regarded as divisible into two suborders termed: 

 o. Nautiloidea. 



b. Ammonoidea. 



TO 



