429 
These diagrams and examples (with the exception of A, B and 
C) were taken from Carboniferous forms published in the Fourth 
Annual Report of the Geological Survey of Texas, 1892, but they 
are applicable to all of the Nautiloidea, provided certain distinc- 
tions be made. The outline expands by growth from an ananepionic 
stage, in this case having the approximate outline of A, and may 
develop into B and C, with decided lateral angles, but in the ephe- 
bic stage may sometimes return to the form of Edaphoceras, C. 
Species of other groups may pass through B’ and, becoming invo- 
lute, take on the outline of B”, and then, if the shell progresses 
still more, it may tend towards forms of H. ~ 
It must, however, be noticed that fossils of such species occurring 
in the earliest geologic period have not, as a rule, even approxi- 
mately well-defined angles, and these being deficient, the zones are 
not apt to be well differentiated. One can readily see that these 
shells, even though they may be involute from an early stage, have 
not the more highly specialized characteristics of the whorl found 
in some of the Devonian fossils. The latter, in their turn, take 
rank as a whole below the still more progressive and highly orna- 
mented nautilian shells of the Carboniferous which represent the 
acme of the order. 
The paracme of the order begins in the Trias, and retrogression 
is plainly manifested in the steady decline of the external ornaments 
and less angulation of the whorl. The universal absence of all of 
the third and fourth orders of modifications of the whorl is one of 
the marked features of this decline, beginning with the Trias and 
becoming universal in the Jura and other subsequent periods. 
The nomenclature of the sutures needs no special description, 
except with reference to the ‘‘annular lobe.’’ This is a small 
indentation in the sutures, occurring either in the centre of a dorsal 
saddle or a dorsal lobe. It is pointed or V-shaped. In some forms 
it may arise before a dorsal lobe is formed in the middle of a primi- 
tive but persistent dorsal saddle, or it may arise subsequently in the 
centre of a broad dorsal lobe. Its development has not been fully 
described. It is often accompanied by an internal pointed caecum 
called the ‘‘ annular cone,’’ and both are probably connected with 
the development of the ‘‘ annular muscle.”’ 
It has been usual to measure the distance of the siphuncle and 
describe its position, with more or less circumlocution, as ventral, 
dorsal, central, etc., but in these descriptions the following terms 
