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changed physical requirements of its surroundings and this position 

 is better supported by facts than any other hypothesis. 



Confining the discussion to the Tetrabranchiata, which are the 

 most favorable for the present purposes, the next problem present- 

 ing itself is whether the two orders, Nautiloidea and Ammonoidea, 

 have had a common origin, or whether they bear internal evidence 

 of having sprung from different ancestors. 



The embryo of all Ammonoidea, as shown by the author in his 

 Embryology of the Fossil Cephalopods of the Museum of Compara- 

 tive Zoology, and since confirmed by the more extensive researches 

 of Dr. Branco, is the little bag-like shell first discovered by Sae- 

 mann. This is attached to the apex of the secondary shell. The 

 embryonic bag has been called the protoconch by Owen ; and the 

 secondary or true shell, the conch. 



There is no protoconch in most Nautiloidea, as first shown by 

 Saemann, then by Barrande, and subsequently by the author and 

 Branco ; but where it ought to have been attached on the apex of 

 the conch, there is a scar, first demonstrated by Barrande. The view 

 brought forward by the author, that this scar indicated the former 

 existence of a protoconch in the Nautiloidea, has been opposed by 

 Barrande, Branco, and several authors, on the ground that the cica- 

 trix demonstrated the existence of a distinct embryonic form. 

 Therefore, according to Barrande, the Nautiloidea were not similar 

 to the Ammonoidea in their earliest stages of growth, and must 

 have been equally distinct in origin. 



I have found the protoconch in several forms of Orthoceratites, 

 the figures being reproduced here, Figs. 3-7, and, further, it can 

 probably be found on the apex of all of the so-called perfect shells, 

 which have no scar or cicatrix. These, when described by De- 

 Koninck, were supposed by him, in his " Calcaire carbonifere " 

 {Ann. du mus. roy. de Belgique), to be fatal to this conclusion. 

 Having no scar, they could not possibly, according to DeKoninck, 

 have had a protoconch. When the so-called perfect apex is broken 

 off, the observer will probably find that this apex was the shriveled 

 remains of a protoconch which concealed the cicatrix underneath, 

 as in Fig. 4. 



There is therefore no essential difference between the embryos of 

 the Ammonoidea and those of the Nautiloidea. There are some of 

 minor importance which we cannot discuss here. These, however, 

 do not interfere with the facts of general agreement ; and there is 



