PHYLLOCERAS ZETES. 423 



characters. Professor Qnensted described these Ammonites as one species, and named 

 what lie considered their varieties by the zones in which they were found, hence we have 

 Phyll. heterophjllus numismalis = Plnjll. Loscombi, Sow., Phi/U. hcterophijllus amalthei 

 z=I>liy]J. Zetes, d'Orbig., Pliyll. Jieteroplii/llus Posidoiiicp = Phi/ll. heteropliyllus, Sow. 



PlnjJloceras Zetes has a discoidal shell, much compressed and extremely involute, the 

 last whorl enveloping nearly the entire disc. The sides are flat, and gently bevelled 

 toward the siphonal area, which is narrow and round; the greatest width of the 

 disc being around the umbilical elevation. The shell is extremely thin, and sculptured 

 with very fine lines of growth, which are straight on the inner and gracefully bi-flexed 

 on the walls of the body-chamber. The siphonal area is narrow and smoothly rounded, 

 and the small syphon lies immediately beneath the shell (fig. 2), where the longitudinal 

 line indicates its position. The umbilicus is narrow from the extreme involution of the 

 spiral, which envelopes four fifths of the lateral surface of the whorls (see fig. 1). The 

 suture-line is extremely complicated, but very beautiful in its tracery, forming nine lobes 

 in its varied convolutions. The siphonal lobe is short and wide, having four lateral 

 branches on each side, the innermost is small, the second and third have several 

 branches, and the fourth or terminal one ends in two lanceolate twigs. The siphonal 

 saddle is wider than the siphonal lobe, and ends in numerous folioles, which are larger 

 on the inner than on the outer side of the saddle, from the root of which supple- 

 mentary lobules arise (see fig. 3). The principal lateral lobe is very large, being much 

 longer and wider than the siphonal, it has an arborescent appearance, havin"- a 

 thick stem with two lateral branches on each side, after which it bifurcates, and gives 

 off two longer branches, the external branch bends over towards the siphon and sends 

 out six branches, three long, extending into the siphonal saddle, and three smaller, 

 bending towards the inner side. The inner terminal branch divides into two branchlets, 

 which send out lanceolate terminations. The outer lateral saddle is long and narrow, 

 and terminates in from six to eight long, narrow folioles. The inner lateral lobe is about 

 the same length, but much narrower than the principal lateral, and composed of a central 

 stem with five lateral branches on each side and a long terminal branch, each dividino' 

 into lanceolate points, having a style of incision much resembling the principal. 

 The inner lateral saddle is much shorter and narrower than the outer lateral saddle, and 

 terminates in three branched folioles ; there are six auxiliary lobes, which much resemble 

 each other in structure in having a solid stem with a number of sharp lanceolate lateral 

 and terminal branches. The lobes diminish in length and width from without inwards, 

 and the six auxiliary saddles have their sides foliated nearly after the same fashion. 



The extremely complicated suture-line of this species produces one of the most 

 beautiful foliated structures that I am acquainted with in Ammonites; a portion 

 is delineated in fig. 3, and is beautifully drawn in Quenstedt's ' Cephalopoden,' tabl. 

 ■vi, fig. 1. 



Affinities and Differences. — This species very much resembles Phylloceras Iietero- 



