CEPHALOPODA. 213 
Of the fossil Zetrabranchiata the most important are the 
Orthocerata and the Ammonites. The Orthocerata (fig. 114) 
played a very important part in the seas of the Palaeozoic or 
Ancient-life period of the earth’s history, in which they appa- 
rently filled the place now taken by the predacious cuttle-fishes. 
Fig. 114.—Orthoceras explorator. 1. Side view of a fragment, showing the edges 
of the septa. 2, Transverse section of the same, showing the siphuncle (s). 
They agreed with the Vauzz/us in having a many-chambered 
shell, divided by curved partitions, perforated by a tube or 
siphuncle. The shell, however, differed from that of the Vaz- 
tilus in not being curved or coiled up, but in being straight. 
In other nearly allied forms the shell was bent or even partially 
coiled up, but never so completely as in the true Vautilus. 
Many of the Orthocerata were of small size, but some of them 
were colossal, shells having been found of six or seven feet in 
length, and as thick as the body of a man. 
The Ammonites, with a number of allied forms of varied 
shapes and beautiful structure, appear to have taken the place 
of the Vautilide, to a great extent, in the seas of the Secondary 
period; at which time, too, Dibranchiate Cephalopods first 
made their appearance. The true Ammonites resembled the 
Nautilus in having a many-chambered shell, which was coiled 
up into a spiral, but the position of the siphuncle was different, 
and the partitions or septa between the various chambers of the 
shell were wonderfully folded and lobed instead of being simply 
curved. The numerous beautiful shells allied to the Ammonites 
cannot be even mentioned here; but it is to be remembered 
that they are almost all characteristic of the Secondary period 
in geology, and that they are hardly known as occurring in the 
older period (Palzeozoic epoch). 
