Dd GENESIS OF THE ARIETIDA. 
We pointed out in “ Embryology of Fossil Cephalopods,”' that the loosely 
coiled stages prevalent among Nautilinidee were repeated in the early stages 
of development in some of the Goniatitinae 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 Goniatitina 
and in all Ammonitine 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? have shown that all ot 
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 Goniatitinse 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 Gym- 
nites fecundus, 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 Goniatitine, 
1 See especially articles ‘ Whorls ”’ and “ Umbilicus,’’ Bull. Mus. Comp. Zodl., LENG: 5; 
2 Paleontogr., 1880, 1881, XXVI., XXVI. 
