618 



MOLLUSCA 



PHYLUM VI 



of the conch, that the protoconch is closely enwrapped by the first volution, 

 and no Perforation is visible even under a magnifier. There are two pits, 

 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 Clymeniidae). The bactriticone obviously represents the 

 primitive or primary radical of the Ammonoidea, and the mimoceracone the 

 next or secondary radical of this Order. 



Ammoniticones of the Nautilinidae introduce a peculiar form of volution, 

 the anarcestean (Fig. 1171), which is depressed and crescentic in section, and 

 may be regarded as an ancestral radical. These f orms evolve a series becoming 

 more involute and compressed (Fig. 1172), and some with elevated or narrow 

 venters and well-defined lateral zones (Fig. 1173), but still retaining in the 

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

 radical forms of similar aspect and with somewhat similar sutures (Fig. 1165) ; 

 they then produce a series of compressed discoidal shells having cordiform or 

 quadrate sections (Figs. 1166-1168), and these also develop involute forms 

 among specialised species. The Gephyroceratidae and Beloceratinae have a 

 similar history, but in the Glyphioceratidae coronate or gastrioceran forms with 

 trapezoidal section and broad venter become common in the ephebic stages 

 (Fig. 1196). In the more specialised groups, the anarcestean character re- 

 appears only during the young stages. 



Phylogeronetic series (Bhabdoceras, 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. Obviously the first stage of the conch 



Fig. 1138. 



Asellate protoconch of Gephyroceras calculi- 

 forme (Beyr.). Upper Devon ian ; Büdesheim, 

 Bifel. A, viewed from in front. B, From the 

 side (after Branco). 



Fig. 1139. 



Latisellate stage of Ärcestes cymhiformis (Wulfen). 

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

 B, From the side (after Branco). 



was that of a living Chamber, the protoconch being without internal septa 

 or siphuncle (Fig. 1150). 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 

 septum closed the aperture of the protoconch, and the caecum projected into 

 its interior. The caecum is connected with the internal surface of the proto- 

 conch by bands (Figs. 1150, 1151); or semiconical prolongations, described 

 by Munier-Chalmas as the prosiphon. But these bands are of various shapes, 

 are not connected with the interior of the caecum, and appear to be merely 



