AEGOCERAS BOUCAULTIANUM. 327 



Aegoceras Botjcatjltianum, oV Orbigny. PI. XVIII, figs. 1 — 4. 



Ammonites Boucaultianus, d'Orbigny. Terr. Jurass., vol. i, p. 294, pis. xc, xcvii, 



1842. 



— — Dumortier. Depots Jurassiques du Bassin du Rhone, 



vol. ii, figs. 3 — 5, p. 138, pi. xxxix, figs. 1, 2, 186/. 



— — Collenot. Geologique de l'Auxois, p. 237, 1873. 



Diagnosis. — Shell large, discoidal, and much compressed ; whorls slightly convex and 

 largely involute ; umbilicus very narrow, inner whorls nearly all concealed and covered 

 with a number of small biflexed ribs, unequally bifurcate near the umbilical margin, and 

 terminating at the outer margin in a series of small bead-like tubercles ; siphonal area 

 narrow, and convex, sides with the rows of tubercles derived from the terminations of the 

 small costal lines ; aperture narrowly oblong ; lobe-line extremely tortuous, with a great 

 development of the siphonal and principal lateral lobes. 



Dimensions. Figured specimen (fig. 1) : — Diameter 155 millimetres; height of the 

 last whorl near the aperture 90 millimetres ; width of the umbilicus 24 millimetres. 



Professor d' Orbigny' s specimen : — Diameter 120 millimetres. Proportional measure- 

 ments in relation to the diameter: — width of the last whorl yp-; involution of the last 



28 

 100- 



whorl -yqq ; width of the umbilicus -j^j ; thickness of the last whorl 



Description. — The shell of this rare Ammonite is discoidal and much compressed. 

 The whorls are slightly convex and ornamented with numerous narrow ribs, which* 

 commencing at the umbilical margin, first bend backwards, then incline forwards, and 

 at about the middle of the whorl bifurcate, then sweeping towards the aperture they 

 terminate at the side of the outer margin in a small tubercle, and, without becoming 

 entirely interrupted, pass across the area to the opposite side (fig. 3). The siphonal area 

 is very narrow, slightly convex, feebly ribbed in the middle, and bounded on each side 

 by a row of tubercles, which are developed on the terminal ends of the ribs (figs. 

 2 and 3). In the cast the area is rounded, but when the shell is preserved there is a 

 median depression, with rows of lateral tubercles corresponding to the terminations of 

 the ribs (see figs. 2 and 3). The spire is formed of whorls, which largely embrace each 

 other ; the last whorl is so extremely involute that it almost completely envelopes the 

 penultimate whorl, and nearly conceals the umbilicus (figs. 1 and 2). 



The aperture is narrow, elongated, and much compressed (fig. 3) ; the lobe-line is 

 extremely tortuous, and d'Orbigny, who had the opportunity of seeing and figuring 

 the septa, has given the following description : — The septa are symmetrical, divided 

 on each side into four very complicated lobes, formed of pairs of parts, and cf 

 saddles composed of folioles almost in pairs; the siphonal lobe is as long and almost 

 as wide as the principal lateral lobe, deeply divided into three very ramified and very 

 complicated branches (see PI. XVIII, fig. 4) ; the siphonal saddle, wider than the 



