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an umbilical perforation which is of medium size and has also 

 slower growth of the metanepionic and paranepionic substage than 

 most of the shells of this period. This species, figured on PI. xi, 

 Figs. 32 and 33, has so large a perforation and so gradual an in- 

 crease in bulk of the nepionic, that it affords no basis for a belief 

 in mechanical causes. If it had been found that the dorsal furrow 

 occurred a little later or not at all in this specimen then there 

 might have been some grounds for the supposition that genism had 

 had no influence upon the perpetuation of the impressed zone. 

 But when one finds in place of retardation a slight acceleration in 

 the development of the dorsal furrow the facts certainly appear to 

 be very strong in favor of the ordinary theory of diplogenesis and 

 tachygenesis. 



The same argument applies with greater force to the Nautiloidea 

 found in the Cretacic. These being more remote than Jurassic 

 species from any primitive nautilian forms, they ought to exhibit 

 the action of tachygenesis in the earlier appearance of the dorsal 

 furrow at least in a considerable number of the species. 



From the remarks already made above and from the figures 

 given, especially on Pis. xii and xiii of this work, it may be seen 

 that so far no specimen has been found in this period which did 

 not show the presence of a dorsal furrow on the metanepionic volu- 

 tion, a substage earlier than most of the species of the Jura. This 

 fact has already been used in other connections, especially in the 

 discussion upon the relations of the dorsum to the venter in nau- 

 tilian shells. It is very positive evidence against the supposition, 

 that the configuration of the dorsum of the metanepionic substage 

 has any effect upon the outline of the dorsum of the paranepionic 

 even in cases where they are brought close together on the opposite 

 sides of even the narrowest of umbilical perforations. Provided it 

 did not touch it is obvious that the dorsal side of the paranepionic 

 substage in Cretacic shells was free to assume any shape.* 



In following the same theoretical line into the Tertiaries, the 

 evidence is less satisfactory; only one species was found, Eutrepho- 

 ceras imperialis, which gave any evidence. This had the dorsal 

 furrow in the metanepionic substage. The Aturidse, however, 



*It will be easily seen that this argument could also be applied to the case ol Trocho- 

 ids canadensis, but in the absence of positive evidence in the genetic series of the 

 Tarphyceratidse I have thought it best not to assume that such use could be made of the 

 parallel facts observed in Mesozoic shells. 



