HARPOCERAS SERPENTINUM. 435 



rugose, highly-flexed ribs (fig. 1), which proceed from theumbihcal margin, pass forward, 

 and at about two-fifths of their length form a prominent curve backwards ; then again 

 turn forwards in a graceful bend towards the siphonal area, sweeping away towards the 

 aperture by the side of the subacute keel (fig. 2). The whorls are flattened, and about 

 one fourth involute, down the middle of the sides is a shallow depression or lateral 

 sulcus, into which the curved portion of the ribs dips ; the siphonal area is narrow, and 

 formed by the forward bending of the ribs, and the straight, prominent, elevated keel, 

 which is not distinct from the ribs, but appears to be formed by their union. 

 The spire is composed of five or six whorls, which are all fully exposed in the wide 

 umbilicus, the amount of involution being about one fourth part of their height which 

 leaves all the inner volutions fully exposed. The spiral margin of the whorls is very 

 acute, abrupt, and angular, imparting a remarkable stair-like character to the umbilicus 

 of this species. 



The aperture is oblong and compressed ; the sides have tvifo lateral lappets as shown 

 in the woodcut (Fig. 194), and the abdominal portion extends outwards in the form of 

 a long projecting spine, intimately associated with the lateral curving forward and 

 retreating backward of the inflected ribs. 



The suture-line is very complicated (figs. 1 and 3) ; the siphonal lobe is narrow, and 

 much shorter than the superior lateral, it is ornamented on each side with five digitations, 

 and is larger at the base than at the summit of the lobe. 



The siphonal saddle is two-fifths wider than the principal lateral lobe, and is divided 

 into two unequal portions, by a long accessory lobe, the internal being the largest, much 

 resembling in size and structure the inner lateral lobe. Each of the compartments of 

 the siphonal saddle terminates in six or seven folioles. 



The principal lateral lobe has a large complicated structure, having a thick stem, 

 ornamented on each side with five or six long digitations, the internal branches being 

 the longest, with a large terminal tuft at the summit. 



The lateral saddle is much smaller than the principal lateral lobe, and has a very 

 irregular form from the unequal size of its folioles. The internal lateral lobe is much 

 smaller than the principal lateral, being about one third the width and one half the lengtli 

 of that large lobe. It has a very irregular form, with two lateral and one terminal bifid 

 branches. 



The first auxiliary saddle, having a similar form, is about one half the size of the 

 lateral saddle. The two auxihary lobes are small, the largest resembles in miniature the 

 principal, and the smaller is simple and bifid. The chief character to be noted in the 

 suture-line of this species is the excessive development of the principal lateral lobe and the 

 great size of the siphonal saddle, with its long accessory lobule. This structure is very 

 fairly delineated in the suture-line drawn in sitii on the side of fig. 1 and in the outline 

 of fig. 3. 



Affinities and Differences. — This Ammonite resembles Harpoceras radians, Harpoceras 



