536 MOLLUSCA SUB-KINGDOM VI 
Order 2. AMMONOIDEA.' 
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 hyponomic sinuses. Sutures, as a rule, have ventral 
lobes in the later stages of ontogeny ; the inflections become more numerous than im 
Nautiloids even in Palaeozoic genera, and their outlines during the Mesozoic are 
extremely complex.  Siphuncle invariably small, and (except in Gastrocampylt) 
situated near the venter. Funnels short, monochoanitic in primitive forms, but 
becoming diplochoanitic during the Palaeozoic, and as a rule, chloiochoanitie 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aperture was closed when the animal was retracted by a single horny plate 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 Lactrites 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 bactriticones (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 q@mimoniticones. The 
term forticone, 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, 
S. S., 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.—Ovrick, 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.—Haug, £., 
Etudes sur les Goniatites (Mém. Soc. Géol. France, Paleont., VII.), 1898.—Zevi, G., Fossili degli 
strati a Terebratula aspasia (Boll. Soc. Geol. Italia, XV.), 1895.—Parona, C. F., and Bonarelli, G., 
_ Faune du Callovien inférieur (Chanazien) de Savoie (Mém. Acad. Savoie, VI.), 1897.—Semenoff, B., 
Anwendung der statistischen Methode zum Studium der Vertheilung der Ammoniten (Ann. Geol. 
- Mineral. Russie, II.), 1897.—Smith, J. P., Development of Lytoceras and Phylloceras (Proc. Calif. 
Acad. Sci. [3], I.), 1898.—Choffat, P., Les Ammonées du Bellasien, des Conches 4 Neolobites Vibra- 
yeanus, du Turonien et du Sénonien. Faune crét. 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. 
es 
