do2 
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. 
s  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. 
