Fig. 256.—Diagram of the sutures of Ammonites: A, an elaborate suture (Phylloceras); B, a simple suture (Ceratites); s.s, siphonal, s.v, ventral, s.l, first lateral, s.l´, second lateral saddles; s.a, s.a, auxiliary saddles; l.v, ventral, l, first lateral, l´, second lateral lobe; l.a, l.a, auxiliary lobes. The arrow points towards the aperture. (From Woodward.) Compare Fig. [258].
The sutural line, which indicates the septa, and is generally concealed beneath the outer layer of shell, consists of a number of lobes or depressions, the concave part of which is directed towards the aperture. Between these lobes lie corresponding elevations, or saddles, the convex part of which is directed towards the aperture. There are six principal lobes (Fig. [256]): the siphonal or ventral, which is traversed by the siphuncle, the dorsal, and a superior and inferior lateral on each side; smaller auxiliary lobes may succeed these latter. The adjacent saddles have received corresponding names. As a rule the sutural line is very complex, but in some cases (Goniatites, Lobites) it is simple (Fig. [258], A). The first saddle of a large number of genera serves as a means of classification, according as it is broad or narrow. Some authorities reverse the terms ventral and dorsal, as applied above. It is probable, however, that the position of the animal of Ammonites in its shell resembled that of Nautilus. The siphuncle is dorsal (internal) in Clymenia only, ventral (external) in all other genera.
The aptychus of Ammonoidea is a corneous or calcareous valve-like body, generally formed of two symmetrical parts (Fig. [257]). It has been regarded by some as the covering of the nidamental gland, and hence as occurring only in the female, by others, with more probability, as an operculum, covering or imbedded in a hood formed, as in Nautilus, of modified arms. Sometimes the Aptychus is in a single piece (Anaptychus), sometimes the two pieces are united on the median line (Synaptychus).
Fig. 257.—Aptychus of Ammonite (Trigonellites latus). Kimmeridge Clay, Ely. × ½.
The Ammonoidea are thus classified by Dr. P. Fischer:—
| (a) Retrosiphonata | Goniatitidae. | ||
| (b) Prosiphonata | No Aptychus or Anaptychus corneous, single | First saddle, wide | Arcestidae, Tropitidae, Ceratitidae, Clydonitidae. |
| First saddle, narrow | Pinacoceratidae, Amaltheidae, Ammonitidae, Lytoceratidae. | ||
| Aptychus calcareous, valves double or united | Harpoceratidae, Stephanoceratidae. | ||
(a) Retrosiphonata. Fam. 1. Goniatitidae.—Shell nautiloid, whorls sometimes disjoined, siphuncle ventral or dorsal, sutures simple. Principal genera: Clymenia, Goniatites (Fig. [258], A).—Devonian to Carboniferous.
(b) Prosiphonata. Fam. 2. Arcestidae.—Shell globular, smooth or striated and rayed, body-chamber very long, aperture often with a projecting hood, umbilicus closed by a callosity, lobes numerous, foliaceous, aptychus present. Principal genera: Arcestes, Lobites.—Principally Trias.
Fam. 3. Tropitidae.—Differs from Arcestidae mainly in the more highly ornamented surface, which is decorated with ribs which become granular at the periphery. Principal genus, Tropites.—Trias and Lias.