EVOLUTION 111 
the close of the Cretaceous period the whole group 
died out after experimenting in every type of shell- 
form in the effort to survive. 
Nothing is at present known of the embryonic 
development of Nautilus, and we do not, consequently, 
know if the primitive embryo shell differs in any 
respect from the adult; but the fact that the earliest 
Cephalopods had straight shells, and that the line of 
development led through curved to coiled forms is 
suggestive, and raises the speculation whether the 
primitive Gastropod shell may not also have been 
straight, and this phase in its development sub- 
sequently suppressed in its embryonic history. 
Following up the scale of geological time, we meet 
with the first of the Decapods (A ulacoceras, belonging 
to the family Belemnoteuthide) in the Trias. It is 
interesting to note that, in the same series, the 
earliest Gastropod referable to the Tectibranchia, a 
species of Bullinella, is also recorded, so that we 
have a Cephalopod with an internal shell comparing 
in time with a Gastropod of a group that only sub- 
sequently in the Chalk period achieved a partially 
internal shell (Philine). 
The Myopsida, or next higher tribe of Cephalopoda, 
began in the Lias (Geoteuthis and Beloteuthis), while 
in the Cretaceous of Lebanon the oldest known 
Octopod, Palgoctopus Newboldi, makes its appearance 
just as the Belemnites and Ammonites disappear 
from the scene. 
