16 GENESIS OF THE ARIETID. 
the Asiphonophora, which may have been more of a Pteropod or Scaphopod 
than a Cephalopod. So far as the shell goes, there are no similarities to the 
peculiar shell of Dentalium, but perhaps more to that of a Pteropod. 
The next step in this line of genesis must have been the ancestral generator 
of the characters of the czcosiphonula, which we propose to call the Caecophora, 
a form which must have been a reality in some shape, and in some species 
doubtless had the characters of the ccosiphonula in its ephebolic stages. 
This class of forms, though having septa and a central axis, which we might 
have to consider as a primitive siphon, was nevertheless quite distinct from 
those which followed. 
The next link in the genealogical tree must have been the ancestor of the 
peculiarities of the macrosiphonula, and this is luckily a well known form. 
The Endoceratide enable us not only to see that the previous train of induc- 
tion is legitimate, but to connect our line of hypothetical forms with the next 
in the evolution of the group. The Endoceratidee are true Macrosiphonophora, 
according to the nomenclature adopted here, and are transitional to the more 
highly specialized and stable modification which had what we have termed the 
microsiphon. 
When this organ came into being in the direct line of change, the evolution 
of the forms also changed its character. The more rapid or accelerated modes 
of change were replaced by slower processes. The changes occurring in the 
types preceding, and including the Endoceratidse (Macrosiphonophora), were, 
if we can judge by the abrupt transitions of the genera in this family, more 
rapid and more important in their effects on structures than was the rule 
subsequently. This is also shown in the structural changes taking place in 
the embryos of Nautiloids and Ammonoids, as compared with the slow and 
comparatively slight changes of the subsequent stages of growth. The rapid 
acceleration of the macrosiphonulate character during the evolution of the 
Endoceratidz, the still more rapid acceleration which took place in the evo- 
lution of the microsiphon among Ammonoids, and the fusion, through accelera- 
tion in development, of the characters of the asiphonula with the protoconch, all 
bear witness to the truth of this induction.’ 
The neepionic stages in ancient asellate forms of the Ammonitine, as has 
been shown above, may be considered as indicating the primitive radical, the 
straight orthoceran, and the gyroceran, or loosely coiled nautilian shells; but in 
1 We have already traced the more rapid evolution of the ancient forms of Cephalopoda, and need not 
go into the matter any further in this monograph than to state that these facts accord with the law an- 
nounced in Genera of Fossil Cephalopods (Proc. Bost. Soc. Nat. Hist., XXII. p. 262), which reads as 
follows: ‘‘ These facts, and the acknowledged sudden appearance of all the distinct types of Invertebrata 
in the Paleozoic, and of the greater number of all existing and fossil types before the expiration of paleo- 
zoic time, speak strongly for the quicker evolution of forms in the Paleozoic, and indicate a gen>ral law 
of evolution. This, we think, can be formulated as follows: types are evolved more quickly, and exhibit greater 
structural differences between genetic groups of the same stock, while still near their point of origin than they do 
subsequently. The variations or differences may take place quickly in the fundamental structural charac- 
teristics, and even embryos may become different when in the earliest period, but subsequently only more 
superficial structures become subject to great variations.’’ See also Foss. Ceph. Mus. Comp., in Proc. Am. 
Assoc. Ady. Sci., XX XII., 1883, p. 338, 
