ORDER II 



AMMONOIDKA 



587 



;iiid no perforation is visible even under a magnifier. There are two pit.-, 

 however, one on either side of the apical end of the conch, which remain as 

 remnants of this perforation, and are present in all ammoniticones (except 

 perhaps certain Gastrocampyli}. The bactriticone obviously represents the 

 primitive or primary radical of the Ammuni<lt. and the niimnrrrarone the 

 next or secondary radical of this order. 



Ammoniticones of the J//V/vw //,////// introduce a peculiar form of volut inn. 

 the anarcestean (Fig. 1122), which is depressed and crescentic in section, and 

 may be regarded as the Tertiary radical. These forms evolve a series becoming 

 more involute and compressed (Fig. 1123), and some with elevated or narrow 

 venters and well-defined lateral zones (Fig. 1124), but still retaining in the 

 young more or less of the anarcestean aspect. The Gastrocampyli have a few- 

 radical forms of similar aspect and with somewhat similar sutures (Fig. 1116) ; 

 they then produce a series of compressed discoidal shells having cordifoi m or 

 quadrate sections (Figs. 1117-1119), and these also develop involute forms 

 among specialised species. The Mesocampyli and Glossocampyli have a similar 

 history, but in the Eurycampyli coronate or gastrioceran forms (representing a 

 quaternary radical) with trapezoidal section and broad venter become common 

 in the ephebic stages (Fig. 1132). The primitive radical forms of the 

 anarcestean ammoniticones are replaced among the Phyllocampyli by the 

 smooth, discoidal, compressed quinary radical, or prolecanitean type (Fig. 1155). 

 This appears among the adult Gastrocampyli, although with distinct sutures, 

 and is also found among the Discocampyli. In the more specialised sub-orders, 

 the anarcestean type reappears only during the young stages. These last two 



FIG. 1080. 



Asellate protoconch of Gephyroceras calculi- 

 forme, Beyr. sp. Upper Devonian ; Biidesheim, 

 Eifel. A, Viewed from in front. Ji, From the 

 side (after Branco). 



Bio. 1090. 



Latisellate stage of Arcestes cymbiformis, Wulfen sp. 

 Trias ; Aussee, Austria. A, Viewed from in front. 

 /;, From the side (after Branco). 



radicals persist into the Jura and Cretaceous, but exhibit more complex sutures 

 and other progressive structural changes in their ephebic stages. 



Phylogerontic series (Ehabdoceras, Cochloceras) make their appearance in the 

 Upper Trias, become more abundant in the Jura, and still more important 

 during the Cretaceous. They have their own peculiar radicals, sometimes 

 found among discoidal and again among more involute shells, but for the 

 most part they do not originate from smooth shells. 



The same descriptive terms are used for shell characters among Ammonoids 

 as among Nautiloids (see p. 512 et seq.). Obviously the first stage of the 

 conch was that of a living chamber, the protoconch being without internal 

 septa or siphuncle (Fig. 1101). Then, after building out the usually flattened 

 neck or apical part of the conch, the animal rested, and the first septum as 

 well as the caecum (or incipient stage of the siphuncle) was secreted. The first 



