ig8 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 an egg. 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 

 egg substance as a 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 parent 

 of the egg, no other data are available than its form. For a 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 Anatidae 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 

 Anatida 1 , while they differed to a marked degree from those of other 

 families. These similarities and differences are illustrated in the 

 eggs shown in PI. XXI. The eggs there shown were selected to 

 represent as nearly as possible types having the general oological 

 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 T ^ of an inch. The 

 measurement of length was therefore made on this completed egg." 



