Present distribution of the so called Ratitae. 171 



constituting the greater part of the thickness of the shell: this 

 substance is transparent in thin sections but in the Ratitae at least 

 it seems to be divided into numerous layers by fine laminae of 

 opaque material arranged parallel to the outer surface of the egg. 

 The inner portion of the shell (the mammillary layer of Nathusius) 

 seems to be composed of much the same material as the last but 

 instead of being continuous it is divided into innumerable small 

 columns separated from one another by narrow cracks and small 

 openings, and terminated at the inner surface of the shell in 

 rounded knobs which are in close contact with the egg membrane. 

 Each column may be more or less completely divided into secon- 

 dary columns by dark vertical lines which tend to converge and 

 curve towards the inner end of the main column. In Struthio this 

 subdivision of the main columns is especially distinct, and each 

 of the secondary columns may terminate in a small convexity. 



The columns like the layer external to them are divided 

 transversely by fine opaque laminae, which may either cross 

 the columns uninterruptedly and parallel to the laminae in the 

 outer part of the shell, or in each column or secondary column 

 may be parallel to one another and strongly convex outwards, 

 and as it were concentric to the inner ends of the columns or 

 mammillae: this structure is best seen in the egg-shell of 

 Struthio. The whole shell is traversed by pore-canals which 

 originate in the spaces between the columns of the mammillary 

 layer and run to the outer surface of the shell, either as simple 

 straight tubes, or with more or less repeated branching as they 

 approach the surface, where they open as small pores, the arrange- 

 ment of which is of considerable importance in determining the 

 affinities of the egg. 



In our fossil the outer membrane seems to have completely 

 disappeared, while the spongy layer is much like that seen in other 

 Struthious birds, particularly Struthio and Aepyoimis, though 

 owing to the mineralisation of the specimens the sections are 

 somewhat obscure, and there is an irregular zone of opaque 

 material just external to the outer ends of the columns. 



The columns themselves are incomplete at their inner ends, 

 having been worn away by the drift sand to which the shell has 

 been exposed, but the portion remaining is similar to the 

 corresponding part of the egg-shell of Struthio, and to some degree 

 of that of Aepyornis, rather than to those of the other Ratitae. 

 Its similarity to the egg of Struthio depends on the tendency to 

 the division of the main columns into secondary ones, and the 

 arrangement of the dark laminae in the accessory columns, in 

 each of which they are convex outwards and concentric round the 

 inner ends of the columns. In Aepyornis the columns are less 

 divided and. the dark laminae seem to run straight across them. 

 In Bhea and Dinornis the columns are less divided than in our 



