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larity with which the law of tachygenesis works in producing the 

 replacement of hereditary characters in every series of forms, and 

 do not trust or know how to use this law. 



The paranepionic substage is consequently among Nautiloids as 

 among Ammonoids of longer duration than either of the preceding 

 substages and of more variable limits. The siphuncle has acquired 

 its ephebic aspect and characters, but it is very often in a different 

 position from that which it subsequently assumes, as it is in Nau- 

 tilus pompilius and other forms figured in this memoir. I have 

 hitherto considered that it included the latter part of the cyrtoceran 

 volution, but it now seems more natural to limit it to that portion 

 of the whorl which assumes the gyroceran curve or, in other words, 

 turns sharply away from the straighter cone of the preceding sub- 

 stages on its return curve towards the apex. This is well shown in 

 Mr. Brooks' drawings and also in the other forms of nautilian 

 shells, especially those of -Barrandeoceras tyrannum and Sacheri 

 of the Silurian. At or near the end of the paranepionic substage 

 in Nautilus umbilicalus and ■pompilius there is in almost every shell 

 a more or less sharply defined constriction which marks a perma- 

 nent aperture. The limits of both substages are subject to varia- 

 tions that will be noticed in the succeeding descriptions, but it 

 suffices here to note the fact that the upper limits of the paranepionic 

 substage are in a general way definable by the limits of the gyro- 

 ceran form in close-coiled nautilian shells. That is to say, this 

 substage, as a general rule, approaches its end and neanic charac- 

 teristics begin to appear at or near the completion of the first volu- 

 tion, when growth brings the whorl in contact with the apex or 

 dorsal side of the conch. Tachygenic forms are often notable 

 exceptions to this definition and introduce modifications that have 

 to be studied in each separate series. 



The transformations that distinguish the subdivisions of the 

 neanic stage are very well marked in some forms and less distinctly 

 in others, but I have constantly found the need of defining two 

 stages. Ananeanic is a suitable term for the first substage, which is 

 usually well marked in nautilian* shells by the first appearance of 



*In my Genera of Fossil Cephalopods nautilian forms have been defined as those having 

 the whorls in such close contact that the dorsum of the enveloping or later formed whorl 

 is modified, either flattened or bent inwardly along the area of contact, and has what is 

 called an " impressed zone." There are, however, some shells that are difficult to classify. 

 These have the volutions in contact but do not have an impressed zone. Most of them 

 are transitional between gyroceran and nautilian forms and may be placed in either 

 category. 



