16 GENESIS OF THE ARIETID.E. 



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 csecosiphonula, which we propose to call the Cascophora, 

 a form which must have been a reality in some shape, and in some species 

 doubtless had the characters of the csecosiphonula 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 Endoceratidae 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 Endoceratidae 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 Endoceratidae (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 

 Endoceratidae, 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. 1 



The naepionic stages in ancient asellate forms of the Ammonitinae, 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 Invertebrate 

 in the Paleozoic, and of the greater number of all existing and fossil types before the expiration of paleo- 

 zoic tune, speak strongly for the quicker evolution of forms in the Paleozoic, and indicate a general law 

 of evolution. This, we think, can be formulated as follows: types are evolved more quickly, an,! exhibit greater 

 structural dij genetic group, of the same stock, while still war 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. Adv. S,-i , XXXII , 1883, p. 3:?S. 



