HARPOCERAS BIFRONS. 437 



Ammonites bifrons, Bumortier. Depots Jurassiques, partie iv, p. 48, pi. ix, figs. 



1, 2, 187-1. 

 Harpocekas — Newnayr. Zeitscbr. Deutscb. geol. Gesellscb., Bd. xxvii, 



p. 908, 1875. 

 — — Tate and Blake. Yorksbire Lias, p. 307, 1876. 



Diagnosis. — Shell discoidal, compressed ; siphonal area tricarinated and bisulcated ; 

 whorls compressed, shghtly biconvex, with a deep longitudinal sulcus occurring at two- 

 thirds distance from the siphonal area and intersecting the ribs ; ribs strong, obtuse, 

 biflexed, and curved against the line of the sulcus ; whorls one-third involute, outer two- 

 thirds most strongly ribbed ; aperture quadrate, elongate, lateral walls vertical with 

 projecting convex lappets, outer abdominal margin having an elongated process (Pi. 

 LIX, fig. 1). 



Dimensions. — Transverse diameter 155 millimetres ; height of whorl near aperture 

 40 inillimetres ; width of umbilicus SO millimetres ; height of aperture 38 miUimetres; 

 transverse diameter of aperture 30 millimetres ; thickness of shell across the last septum 

 .38 millimetres; length of the body-chamber three-fourths the length of the last whorl. 



Description. — Although it is now two centuries ago since this common Ammonite 

 was first figured by Lister, no such accurate delineation of this beautiful form has ever 

 before been drawn as that given in Plate LIX, which displays the whole external anatomy 

 of the shell, shows the length of the body-chamber, and the processes of the aperture. I 

 collected this specimen a few years ago at Whitby, but I have never found a second 

 example as perfect as this type. 



The shell is discoidal and much compressed ; the siphonal area is tricarinated, the 

 median carina most elevated, the two lateral carinae less so and separated from the median 

 carina by two well-marked grooves ; the whorls are compressed and exhibit at their inner 

 third a deep longitudinal sulcus, which divides the side of the whorl into two convex 

 portions, the outer portion is covered with sixty obtuse convex ribs, the concavities of 

 which are directed towards the aperture ; the inner portion is narrower and more convex, 

 being limited by the sulcus, and shows feeble traces of very small ribs ; it is this portion 

 of the whorl that forms the prominent ornament of the umbilicus (see figures 3 and 4). 

 This style of double ribbing is well developed in the two fragments which, covered with 

 the mother-of-pearl coating of the shell, have preserved their markings in great perfection. 



The aperture is quadrate, elongated, and compressed at the sides, and its transverse 

 section (fig. 2) exhibits at the summit two slight sinuosities, and at the base a grooving 

 formed by the return of the spire; the walls of the aperture (fig. 1) show short convex 

 lateral lappets on each side, and the outer abdominal portion a long projecting process. 

 On this figure likewise is delineated the true position of the last septum, marked by the 

 suture-line, and showing from its position that the body-chamber occupied three fourths 

 of the last whorl. 



Professor d'Orbigny made some instructive observations on the morphology of this 



