,S93.j 121 [Hya«- 
branches springing from the ordinal phylum and forming con- 
stituent i^arts of it at their origin but <liverging above, and each 
having its own independent lines of modifications terminat- 
ing in space and time according to their own mor]»liic cycles. The 
term "branch" could be very appropriately used for these sub- 
orders and they could thus be spoken of as the branches of the 
order. 
The ordei-s are more or less complete cycles of suborders and 
we see no reason why the order, whether thought of in the arti- 
ficial light of the old school or that of the new, is not as applicable 
in paleozoology as in zoology. If a descriptive adjunct implying 
a dynamic character is required, the term "limb" conld be ajipro- 
priately employed. The orders of Cephalopoda can be recognized 
as closely converging in the case of the ammonoids, nautiloids, 
and belemnoids. The sepioids may be convergent with beleni- 
noids, but the exact type from which they may linvc originated has 
not yet been pointed out. 
The ephebic structures, and the embryonic and nepionic cliar- 
acteristics of nautih)ids, ammonoids, and bcK'mnoids present 
strong internal evidence of having been deriveil from a common 
ty|)e which must have been an ancient, straight form of nautiloid 
with a small siphuncle and this si()huncle comi)osed of two parts, 
the funnel and the sheath. 
The Orthoceratidae, therefore, are the only pale(,)zoic ty]>e or 
"trunk" form to which the orders of Cephalopoda, the limbs of 
that genealogical tree, can l)e approximately traced or referred, 
this being the only ancient microsiphonulate group with a straight 
shell. The arguments for this position have been given in detail 
in Genera of fossil cephalopods and in Genesis of the Arietidae, 
and recent investigations have confirmed this opinion. It would 
therefore be appropriate to consider this family as the "trunk" of 
the class of Cephalopoda. 
Orthoceras can in its turn be definitely traced to the Endo- 
ceratidae. Tliis family and the Actinoceratidae, the parallel 
group of the Paleozoic, must have had ])receding ancestral forms 
in the Coecosiphonophora and Protosiplionophora corresponding 
to the coecosiphonula and protosiphonula of the ontogeny. 
These last named two families, therefore, are not trunk forms like 
the Orthoceratidae, or limbs like the orders giving off branches 
in the suborders, but constitute the roots of the trunk 
