4 04 SCIENCE PROGRESS 



and internal structure. Now it is generally agreed that the 

 Ammonites are derived, through such simple coiled forms as 

 Anarcestes (fig. 29) and Mimoceras, from a primitive straight 

 form such as the Devonian and Ordovician Bactrites (fig. 28), 1 

 which in turn may be connected with the straight forms 

 {Orthoceras, fig. 30) of the Nautiloids by means of an inter- 

 mediate type like the long pencil-shaped Protobactrites, which 

 ranges from the Silurian to the Carboniferous. 



All the available evidence points at present to the conclusion 

 that Bactrites is an ancestral primitive genus, and is not descended 

 from a coiled shell. It is therefore quite dissimilar in its origin 

 from the equally straight Baculites, which (as already indicated) 

 has been derived by retrograde development from a closely 

 coiled spiral. As Hyatt 2 has concisely stated, "the straight 

 Bactrites-like young of some forms of Anarcestes, the gyroceran 

 young of others of the Goniatitince and the gyroceran adults and 

 young of Mimoceras, indicate the derivation of the Goniatitince 

 to have been from Silurian straight shells similar to Bactrites, 

 if not directly from that genus itself." The close genetic 

 relation between the coiled Mimoceras and the straight Bactrites 

 was also prophetically foreshadowed by Hyatt, 3 when he described 

 the species of Mimoceras as " separable from Bactrites in no 

 essential characteristic except the presence of a permanent 

 protoconch upon the apex. . . . Their characteristics and the 

 protoconch ally them, however, even more closely with 

 Anarcestes. . . . This evidence appears to need but one more 

 link — the finding of a Bactrites with a globular protoconch." 

 This missing link in the chain of evidence was eventually 

 supplied by Branco, 4 who describes this protoconch (or initial 

 chamber of the shell) as agreeing in shape with that of Mimoceras 

 comprcssum, and differing altogether from that of the Nautiloids. 



Among the Nautiloids a case of unrolling of a somewhat 

 parallel nature to that of the Ammonites may be furnished 



1 Whilst this genus is not yet known to occur in the Silurian, it has been 

 recorded from the Ordovician ; the earliest known species is Barrande's Bactrites 

 Sandbergeri from his Etage D, in its two extreme horizons : viz., in d i (equivalent 

 to the Arenig group) and in d 5 (equivalent to the upper part of the Bala group) ; 

 but its presence has not hitherto been revealed in the intermediate stage. 



1 " Genesis of the Arietidas," Smithsonian Contrib. to Knowledge, 1889, p. 1. 



J Proc. Boston Soc. Nat. Hist. xxii. (1884), p. 309. 



4 " Ueber die Anfangskammer von Bactrites," Zeitschr. deutsch. geol. Gesell., 

 1885, Bd. xxxvii. Heft 1, p. 1. 



