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PODA OF MINNESOTA. 
BY JOHN M. CLARKE. 
INTRODUCTORY. 
The Cephalopoda or “head-footed” mollusks are distinguished from the other 
molluscan groups by the possession of a circlet of long fleshy tentacles or prehensile 
organs arranged about 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 
diversity 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: 
i Tetrabranchiata; 
2. Dibranchiata; 
terms which imply the possession respectively, of four and two gills. 
The tetrabranchiates are typified by the living Nautilus pompilius; the 
dibranchiates by the Loligo, or squid, Sepia, or cuttle-fish. 
The tetrabranchiates were wonderfully abundant throughout the Paleozoic and 
Mesozoic periods of the earth’s history, but are to-day almost extinct, while the 
dibranchiates are the predominant cephalopods 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 swborders termed: 
a. Nautiloidea. 
b. Ammonoidea. 
