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is possible to study the relations of these fossil forms very minutely 

 and with a certainty of possessing a clue to their true relations, 

 which is rarely obtainable even among existing animals. For among 

 these we have only the embryos and young of contemporaneous 

 forms and necessarily lose all relations of succession in time, unless 

 the investigation embraces a prolonged series of experiments or is 

 more or less historical, and even then the facts cannot have a very 

 wide chronological range. 



The class of Cephalopoda has two subclasses, Tetrabranchiata 

 and Dibranchiata. These were established by Richard Owen as 

 orders — a purely technical difference, which does not change 

 in any way the value of the structural distinctions as given by this 

 eminent naturalist. The Tetrabranchiata are shell-covered ; and 

 they are represented by the modern Nautilus, the only existing 

 genus. The Dibranchiata are descendants of the former, but 

 enclosed the shell, and resorbed it in many forms, so that they 

 appear as naked animals. The cuttlefishes, squid, devil-fishes, etc., 

 are existing types. In studying these types, the author has 

 been led to adopt a new method of characterizing the divisions, 

 and besides the old structural distinctions, which are still available, 

 to apply the correlations of habit and structure to the elucidation 

 of some of the ordinal characters. 



The classification adopted is as follows : 

 Class Cephalopoda. 

 Subclass I, Tetrabranchiata. 

 Order, Nautiloidea. 

 " Ammonoidea. 

 Subclass II, Dibranchiata. 

 Order, Belemnoidea. 

 " Sepioidea. 



These four orders converge to one type by intermediate forms, by 

 embryology and development of the shells and internal hard parts, 

 by their morphology and by the possession of a similar embryonic 

 shell, the protoconch, or the cicatrix which is a remnant of the 

 aperture of this stage on the apex of the true shell or conch. 



The class is composed of exclusively aquatic and marine animals, 

 and consequently they breathe with gills. The structures of the 

 orders mentioned coincide with the distinct habitats they respec- 

 tively occupy. 



