536 MOLLUSCA SUB-KINGDOM vi 



Order 2. AMMONOIDEA. 1 



Shells similar to those of Nautiloidea in some primitive Palaeozoic groups, but 

 these give rise to others with more highly ornamented shells, the apertures of which 

 have ventral rostra instead of liyponomic sinuses. Sutures, as a rule, have ventral 

 lobes in the later stages of ontogeny ; the inflections become more numerous than in 

 Nautiloids even in Palaeozoic .genera, and their outlines during the Mesozoic are 

 extremely complex. Siphuncle invariably small, and (except in Gastrocampyli) 

 situated near the venter. Funnels short, monochoanitic in primitive forms, but 

 becoming diplochoanitic during the Palaeozoic, and as a rule, chloiochoanitic during 

 the Mesozoic. 



The ontogeny begins with a calcareous protoconch, the apical stage of the 

 conch being an open neck built in continuation of the permanent aperture of 

 the protoconch. The first septum is concave as in Nautiloids, and sutures are 

 straight or have more or less of a saddle on the venter. Young stages of 

 Mesozoic shells recapitulate the primitive characters of Palaeozoic forms. The 

 aperture was closed when the animal was retracted by a single horny plate 

 (anaptychus) or pair of calcareous plates (aptychus), probably secreted by 

 muscular lobes homologous with the hood in Nautilus. 



Shell Characters. There are apparently no characters, not even the presence 

 of a calcareous protoconch, which can be relied upon to separate Bactrites from 

 the orthoceraconic Nautiloids. Nevertheless, the position of the siphuncle and 

 its peculiar funnels are features which seem to place this form with true 

 Palaeozoic Ammonoids. There is but one series of straight shells among 

 Ammonoids, and these are obviously not the same as orthoceracones, but are 

 more properly called badriticones (Fig. 1120). Similarly, the loosely coiled 

 Mimoceras shells (Fig. 1121) are not gyroceracones, but only their morphic 

 equivalents in a different genetic stock ; hence the term mimoceracone should 

 be substituted for gyroceracone. In the same sense the closely coiled sym- 

 metrical shells, comparable in external aspect and intimate structure with 

 nautilicones, should be described among Ammonoids as ammoniticones. The 

 term torticone, however, can be conveniently applied to both groups, since it 

 does not connote any special structures, but is a general name for all asym- 

 metrical spirals. 



Ammoniticones in many Palaeozoic forms are mimoceracones during 

 nepionic stages, and consequently in later stages a perforation is present 

 passing through the umbilicus as in Nautiloids. However, in most Carboni- 

 ferous and all later ammoniticones, the coiling is so close even at the beginning 

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



1 Besides the works cited on pp. 502-505, the following may be profitably consulted -.Buckman, 

 V. &, Divisions of so-called Jurassic Time (Quar. Journ. Geol. Soc., LIV.), 1898. Clarke, J. M., The 

 Naples Fauna (16th Ann. Rep. N.Y. State Geologist), 1898. Crick, G. C., Muscular Attachment of the 

 Animal to its Shell in Ammonoidea (Trans. Linn. Soc. [2], VII.), 1898. Diener, C., Cephalopoda of 

 the Muschelkalk (Mem. Geol. Survey India, ser. XV., Himalaya Fossils, II.), 1895.ffavg, E., 

 Etudes sur les Goniatites (Mem. Soc. Geol. France, Paleont, VII.), 1898. Levi, G., Fossili degli 

 strati a Terebratula aspasia (Boll. Soc. Geol. Italia, XV.), 1895. Parana, C. F., and Bonarelli, (?., 

 Faune du Callovien inferieur (Chanazien) de Savoie (Mem. Acad. Savoie, VI.), 1897. Semeno/, B., 

 Anwendung der statistischen Methode zum Studium der Vertheiluug der Ammoniten (Ann. Geol. 

 Mineral. Russie, II.), 1&97. Smith, J. P., Development of Lytoceras and Phylloceras (Proc. Calif. 

 Acad. Sci. [3], I.), 1898. Cho/at, P., Les Ammonees du Bellasien, des Conches a Neolobites Vibra- 

 yeanus, du Turonien et du Senonien. Faune cret. du Portugal, vol. I. ser. II., 1898. Jackson, R. T., 

 Localised Stages of Development in Plants and Animals (Mem. Boston Soc. Nat. Hist. V.), 1899. 



