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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 touch. 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 

 (disregarding 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 

 siphimcle. That the geological series of shells indicates the same thing 

 every paleontologist knows perfectly 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. pacific-it m 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 protoronch or phylembryo. It is a smooth, oval, 

 bulbous body, similar to that of all the later ammonites. It probably rep- 



