315 
pression or groove in the dorsum of the whorl, where it rests against the 
venter of the preceding whorl. This is the impressed zone. In the mod- 
ern Nautilus, however, this furrow or impressed zone begins in the early 
nepionic stage, before the whorls have come into contact. This occurs 
also in the nautilian shells of the Carboniferous, Jurassic, Cretaceous and 
Tertiary. 
Of this truly remarkable feature of cephalopod development, Hyatt 
says: “When one ascends in the same genetic series to the more special- 
ized nautilian involved shells this purely acquired character becomes, 
through the action of tachygenesis, forced back, appearing as a rule in 
the nepionic stage before the whorls teuch. It is therefore, in these forms 
entirely independent of the mechanical cause, the pressure of one whorl 
upon another, which first originated it. One need only add that this 
configuration of the dorsum is never found in the adults of any ancient 
and normally uncoiled shells, so far as I know, nor so far as have been 
figured.” (31) 
Without reviewing any of the further interesting details of the on- 
togeny of Nautilus, enough has been said to make it evident that if there 
is any truth in recapitulation, the development of Nautilus would indicate 
(distegarding the protoconchal characters) an ancestral line that con- 
tained, first straight or slightly arched, then loosely coiled, and finally 
closely coiled shells, and that the earliest of these possessed a septate 
siphuncle. That the geological series of shells indicates the same thing 
every paleontologist knows perfectiy well. The development of Nautilus 
also affords one of the most perfect illustrations of the law of tachygene- 
sis, in the earlier inheritance of the impressed zone, known in the whole 
animal kingdom. 
One further illustration, from the Cephalopoda, of the parallelism of 
ontogeny and phylogeny must suffice. This illustration is drawn from the 
genus Placenticeras, one of the complex Ammonites of the Cretaceous. ‘The 
development of this genus has been beautifully worked out by Professor 
J. P. Smith (58). The species P. pacificum comes from the Chico forma- 
tion of the Upper Cretaceous. The following account applies to the de- 
velopment of this species and is drawn from the paper by Smith, cited 
above. 
The earliest shelled stage was probably passed before the animal was 
hatched. This is the protoconch or phylembryo. It is a smooth, oval, 
bulbous body, similar to that of all the later ammonites. It probably rep- 
