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GENESIS OF THE ARIETIDA. 
: & 
INTRODUCTION. 
ORIGIN AND CHARACTERISTICS OF SUBORDERS. 
HE succession of forms among the silurian members of the genus Mimoceras 
indicates that true gyroceran shells occurred among Ammonoidea, differing 
from the similar forms among Nautiloids only in the possession of a globular pro- 
toconch and a small ventral lobe. In some silurian and devonian Anarcestes these 
permanent adult stages are repeated in the development of the young. Those in 
Mimoceras compressum are truly cyrtoceran, or open curves at first; and in others, 
as in a variety of Anarcestes fecundus described by Barrande, they are straight. 
The next stage of growth is a loose-coiled or gyroceran form, like the adult 
of Mimoceras. These stages can only be accounted for as hereditary tendencies 
of growth in a type which is being rapidly changed from a primitive ancestral 
straight form with simple sutures into a close-coiled nautilian shell. 
Branco? describes and figures a specimen of Bactrites with a protoconch 
similar to the very peculiar ovoid protoconch of Mim. compressum. He quotes 
Beyrich, who gave him this specimen, as authority for the view that Bactrites 
is connected with Mimoceras as Baculites is with the normal Ammonoids of the 
Cretaceous. This idea was first published by Quenstedt in his “ Die Cephalo- 
poden,’ and it is quite possible that Bactrites of the Devonian may be a de- 
graded form of Mimoceras, but in that case the latter is also a degraded form of 
Anarcestes, or transitional between it and Bactrites. To establish this proposi- 
tion, forms of Mimoceras and Anarcestes should be produced in which uncoiling 
occurred in adults after a close-coiled stage of growth had been passed through. 
Such degraded forms are common in the Jura and Cretaceous, and enable the 
observer to connect Baculites with the normal coiled Ammonoids of the same 
formations. Whether this be so or not, the straight Bactrites-like young of 
some forms of Anarcestes, the gyroceran young of others of the Goniatitine, 
and the gyroceran adults and young of Mimoceras, indicate the derivation of 
Goniatitinss to have been from silurian straight shells similar to Bactrites, if 
not directly from that genus itself. 
1 Genera Foss. Ceph., pp. 303, 304, 309, Proc. Bost. Soc. Nat. Hist., XXII., 1883. 
2 Zeitsch, Deutsch. Geol. Gesell., XX XVII. p. 1. 
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