605 
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 Pl. 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 Pls. 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, /utrepho- 
ceras tmperialis, which gave any evidence. This had the dorsal 
furrow in the metanepionic substage. The Aturide, however, 
*Tt will be easily seen that this argument could also be applied to the case of Trocho- 
lites canadensis, but in the absence of positive evidence in the genetic series of the 
Tarphyceratide I have thought it best not to assume that such use could be made of the 
parallel facts observed in Mesozoic shells. 
