HARPOCERAS LEVI SON I. 439 



whorls flat on the sides, ribs obtuse, sigmoidal, equal in width to the intervening valleys. 

 Carina thick and slightly elevated, with indistinct furrows on each side ; inner whorls 

 strongly ribbed, outer much smoother; aperture quadrate, with lateral lappets and an 

 abdominal projection. 



Description. — This form is nearly allied to the preceding species. The shell is 

 compressed, discoidal, and carinated. The whorls are narrow, and about one-third 

 involute. The sides are flat, and ornamented with broad, obtuse, sigmoidal ribs, 

 separated by valleys of a like breadth. The ribs are thicker and more prominent in 

 the inner whorls, but the last whorl of the adult shell has very obtuse distant undula- 

 tions, which gradually become by degrees a nearly smooth surface. The inner or spiral 

 side of each whorl is well rounded down at right angles to the previous one, so that the 

 wide umbilicus of this species presents a succession of flat terraces of a stair-like fomi. 

 The siphonal area is round, with a smooth, thick, low carina rising in the middle of two 

 indistinct depressions, one on each side. The evolution of this Ammonite difi'ers from 

 some of its congeneric forms. In early life the suture-line develops on each side three 

 lobes and three saddles composed of single parts. The siphonal lobe, a little narrower 

 and shorter than the principal lateral lobe, is divided by the median line up to one third 

 of its height, and presents only one chief terminal point and several lateral digita- 

 tions. The dorsal saddle very wide, almost twice the size of the principal lateral 

 lobe, is divided by a small accessory lobe into two portions, each of which termi- 

 nates in obtuse shallow folioles. The principal lateral lobe is an elongated mass, 

 ornamented on each side with three or four simple digitations, and terminates in several 

 branches, of which the two central are the longest. The lateral saddle is long and 

 narrow, about half the width of the siphonal ; it lies in the sulcus, and is seen with 

 difficulty ; it terminates in a few obtuse shallow folioles. The internal lateral lobe is 

 small, about one fourth the size of the principal, it bends inwards, and has a few lateral 

 digitations, terminating in a longer lanceolate process. The auxiliary saddle is very 

 short and bilobed. The auxiliary lobe is less than the internal lateral, and is armed 

 with lateral digits and a terminal point ; it lies well down towards the spiral suture on 

 the slope of whorl. 



A radial line stretched from the extremity of the siphonal lobe cuts the three last 

 digitations of the principal lateral lobe, but does not approach any of the others. 



In its morphology this species forms a remarkable contrast to Harjj. bifrons ; when of 

 the diameter of forty-five millimetres, it possesses thick, prominent ribs, has a low keel, and 

 two sulci on the area, and in this condition it very much resembles an Arietites, as shown 

 in PI. LXI, figs. 1 and 2. The next volution exhibits the ribs more obtuse and distant 

 from each other ; and in the following volution the ribs have a highly sigmoidal curve, 

 are well inclined backwards, and then curved forwards upon the area. This style of 

 ribbing is very well shown in the beautiful shell figured in PI. LXI, figs. 5 and 6, from 

 the Upper Lias of Ilminster. The fine large adult shell, which forms the subject of 



