198 FIELD CoLUMBIAN MusEUM—GEOLOGY, VOL. I. 
the process as he describes them at least seem logical and reasonable, 
and may well have brought about petrifaction in the case of anegg. Of 
course it is not held as proved that, in the specimen under considera- 
tion, those portions which I have designated as white and yolk have 
been preserved in their original structure and proportions. While this 
may be the case, it is again quite as likely that the portion which I 
have designated as the yolk represents the shrunken residue of the 
ege substance asa whole. Its appearance, indeed, rather indicates 
this, since the curved plates that have been described resemble 
shrunken membranes. The remainder of the interior may then have 
been filled simply with water at the time petrifaction set in. 
It is more difficult to explain the fact that most of the ‘‘yolk” is 
composed of opal while the rest of the interior is made up of chalce- 
dony: The difference is, however, not essentially greater than that . 
often found to exist in different layers of agates. 
For establishing any conclusions as to the nature of the tly 
of the egg, no other data are available than its form. Fora knowl- 
edge of what this indicated I referred to Mr. Wm. A. Bryan of the 
Museum, who has very kindly furnished the following report: ‘‘ The 
form. of the specimen suggested to me on the first examination that 
it was that of an egg of one of the Anatide or Duck family. Further 
study confirmed this conclusion, the similarity in form to that of eggs 
of Anatine birds being marked and such as to distinguish it from the 
eggs of birds of other families. Measurements of the fossil egg, too, 
showed it to have the same proportions as those of members of the 
Anatide, while they differed to a marked degree from those of other 
families. These similarities and differences are illustrated in the 
eggs shown in Pl. XXI. The eggs there shown were selected to 
represent as nearly as possible types having the general dodlogical 
characters of the different families of birds whose eggs might resem- 
ble the fossil form, or whose habits would lead them to deposit eggs 
‘under conditions favorable for petrifaction. To complete the form 
of the fossil egg for this -purpose, the outline on the broken end was 
filled out with plaster. By this method the form of the egg was 
reproduced, any error being probably within ;4, of aninch. The 
measurement of length was therefore made on this completed egg.” 
