170 Walter Rothschild. 



SO that the time during which they have been exposed is probably 

 comparatively short. Unfortunately the inner surface of the shell 

 having been removed by this means some of the important 

 characters exhibited by this part cannot be observed. 



The difficulty of determination is also increased by the mine- 

 ralised condition in which the specimens are, many of the 

 characters which can be observed in the fresh egg-shell being 

 invisible in these fossils. 



It does not seem possible to determine from what region of 

 the shell these fragments are derived and consequently, one cannot 

 make a really reliable estimate of the size of the egg to Avhich 

 they belong. Their curvature is however slight and this circum- 

 stance taken together with the thickness of the shell indicates 

 that the egg was far larger than the largest modern Ostrich or even 

 than that of Struthiolithus chersonensis, Tn the recent Ostriches 

 the shell rarely exceeds 2 mm. in thickness: the thickest noticed by 

 Nathusius is 2.10 mm. in the case of a Somali Ostrich; of the 

 fossil egg of Struthiolithus the same authority gives the thick- 

 ness as 2.6 — 2.7 mm. and in the egg of a large species of Aepyornis 

 in the Musée d'Histoire Naturelle, Paris it is 3.65 mm. and in 

 another large Aepyornis egg in the British Museum 4.4 mm. In 

 our fossil the thickness is 3.2 — 3.4 mm. as now preserved, so that 

 in this respect it is intermediate between the egg of Struthiolithus 

 and that of Aepyornis titan. The dimensions of the former are 

 t8 cm. long diameter, 15 cm. short diameter; in the Paris specimen 

 of Aepyornis the same measurements are 34 cm. and 22.5 cm. 

 respectively, so that it may be estimated that the Algierian fossil 

 egg may have measured as much 25 cm. in its long diameter 

 and 19 cm. in the transverse direction. No living bird can have 

 laid an egg of this size, and the only fossil Ratite from Africa 

 is Eremopezus eocaenus Andrews, from the Upper Eocene beds 

 of the Fayum, Egypt, and this was only about the size of a small 

 Ostrich. We are thus compelled to admit that this region must 

 have been inhabited at some period by a species of bird exceeding 

 in size all know^n species of ratite birds, with the exception of the 

 larger species of Aepyornis. The period at which this bird lived 

 is quite uncertain because at present there are no data, geological 

 or other, which supply any information on this point. 



Turning now to the microscopical characters of this egg-shell, 

 we meet with some interesting points. For a knowledge of the 

 structure of the shell of eggs, particularly of those of the Ratitae, 

 we are mainly indebted to Nathusius, and sections of our spe- 

 cimen have been examined in the light of his figures and 

 descriptions, and have furthermore been compared with actual 

 sections of various egg-shells. Speaking generally egg-shell 

 consists of an outer membrane which in the Ratitae is hard and 

 brittle: within this a thick layer consisting of calcified material, 



