The terminology used to describe the lower jaws of ammonites is that of Kanie (1982), Tanabe (1983), and Tanabe and Fukuda (1987), and illustrated in Fig. 13.3. We employ the terms anterior, posterior, ventral, dorsal, left, and right, to refer to the jaws as they were oriented in life. The jaws are illustrated with the anterior end on top. The most recently formed portion of the jaw is at the posterior end. The two symmetric halves of the lower jaw are called the wings, with the hinge line (symphysis = commissure) along the midline.

Fig. 13.3 Terminology and measurements of the lower jaw of baculites, showing the ventral surface covered with an aptychus.

Most of the jaws we describe are incomplete. We measured the width and length of each wing of the jaw, irrespective of curvature, following the approach of Kanie (1982) and Tanabe and Fukuda (1987). These measurements represent the maximum dimensions of the wing perpendicular and parallel to the symphysis, respectively. Width is a more reliable indicator of size than length, because the long end of the jaw is commonly broken, especially in isolated elements. The width of the jaw equals twice the width of the wings. The ratio of jaw width to length provides an approximation of jaw shape.

Landman et al. (2006) and Landman and Grebneff (2006) discussed the terminology surrounding aptychi. They defined an aptychus type lower jaw as a lower jaw bearing an aptychus. An aptychus is the entire calcareous layer covering the ventral surface of the lower jaw. It consists of a pair of left and right valves. (For additional discussions about the terminology used to describe aptychi, see Trauth, 1927-1936, 1938; Arkell, 1957; Moore and Slyvester-Bradley, 1957; and Farinacci et al., 1976).

Trauth (1927) assigned the coarsely ornamented aptychi of Baculites to the form genus Rugaptychus, and erected parataxa with binomial names written in italics, to describe variation within this morphotype. Moore and Sylvester-Bradley (1957:

L469) characterized Rugaptychus as follows: "Elongate diaptychi [pair of valves] with strong sharp ridges on outer surface, ridges characteristically arranged with angulated bend; inner surface with growth lines, nearly smooth." We follow the practice of Engeser and Keupp (2002) in employing the term rugaptychus as a common name without italics to refer to this kind of aptychus.

Specimens are reposited in the American Museum of Natural History (AMNH), New York; the Black Hills Museum of Natural History (BHMNH), Hill City, South Dakota; and the Sternberg Museum of Natural History (FHSM), Hays, Kansas; and the US National Museum (USNM), Washington, DC.

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