2 GENESIS OE THE AEIETID.E. 



We pointed out in " Embryology of Fossil Cephalopods," : that the loosely 

 coiled stages prevalent among Nautilinidae were repeated in the early stages 

 of development in some of the Goniatitinse and in the later Ammonoids. This 

 repetition was indicated by the form of the embryo which was flattened and 

 depressed, and also in the first sutures and in the embryonal umbilici. These 

 last are two conical or flattened depressions on either side of the protoconch, at 

 its junction with the apex of the conch. They were accounted for as remnants 

 of the umbilical perforation found in the young and adults of Mimoceras and 

 all coiled Nautiloids. 



In our "Genera of Fossil Cephalopods" we narrowed this generalization 

 by comparing the first whorl of the embryo in the close-coiled Goniatitince 

 and in all Ammonitinae with Anarcestes, thus bringing the affinities of all the 

 Ammonoidea to a focus in the silurian genus Anarcestes. These and other 

 similar observations, published before and since the work quoted above, have 

 been founded upon the law of acceleration formulated also in the Preface of this 

 monograph, pp. v, vi, Art. 11 and 14. 



Dr. Branco's extensive and accurate researches 2 have shown that all of 

 these opinions, though founded upon a few specimens only, were sound, and 

 that the law of acceleration can be relied upon as a working hypothesis. Though 

 treating us otherwise with more than just appreciation, this author failed to 

 notice that we had used the law of acceleration in development, or made our 

 inductions with the view of demonstrating its truth as a working hypothesis, 

 and consequently attributed the discovery of this law to Wiirtenberger. 



Among Nautiloids the straight shells in each series appeared first ; they were 

 succeeded by the cyrtoceran, gyroceran, and close-coiled. Among Ammonoids 

 there is only one series — Bactrites, Mimoceras, and Anarcestes — which is 

 parallel with any one series of the many occurring among Nautiloids. The 

 open-whorled stages of the young of Anarcestes and other Goniatitinoe repre- 

 sent a transitional and highly accelerated development. This transitional 

 character is also indicated by the fact that, except in Mimoceras and some 

 species of Anarcestes, the occurrence of the gyroceran form, even in the young, 

 is sporadic. It occurs, as demonstrated by Barrande, in one variety of Gijm- 

 nites fecnndus, and not in the other. Sandberger has shown similar though less 

 marked variations in the young of Anar. subnautilinus, and Branco has described 

 the embryo of var. vittiger of the same species as close-coiled. Other examples 

 might be given, but it only remains to notice Branco's doubts of the accuracy 

 of our drawings of the young of Gon. atratus and Gon. Listeri. Both of these 

 were found by him to belong to his close-coiled division of the Asellati of 

 the Carboniferous. Our drawings were made with a camera. The details they 

 contain show, better than any defence we can make, that they were also 

 closely studied by the author, and often corrected before being placed upon 

 stone. They indicate that primitive gyroceran forms of young are occasionally 

 found even among the highest forms of carboniferous Goniatitinte. 



1 See especially articles " Whorls " and " Umbilicus," Bull. Mus. Comp. ZoSL, III. No. 5. 



2 Paleontogr., 1380, 1881, XXVI., XXVII. 



