Ammonites of the Mediterranean and Juvavian Trias. 465 



furrow in the centre of the convex portion, and a long one-pointed 

 antisiphonal lobe. This furrow is frequently limited by smooth or 

 knobly carina?. In some forms the ribs end in a thickening near 

 the furrow. A number of dichotomous or simple ribs, all of them 

 beginning at umbilical knobs, cover the sides, on which there are 

 also rows of knobs. The forms of higher geological age have high 

 saddles, with entire margins; in others of less age, from the "St.- 

 Cassian " strata, the notch extends over the heads of the saddles. 



In Arp. modestus, an aberrant form, standing next to Arp. Laubei, 

 the ribs unite over the convex portion. 



Trachyceras, Laube. — The sculpture extends without interruption 

 as far as on the convex portion, in the centre of which it is always 

 interrupted by a narrow interval, close to which the Mediterranean 

 forms show constantly one or more ranges of spines. In the 

 Juvavian forms, minute notches at the ends of the ribs (see Trachyc. 

 bi-crenatum, Hauer) or notched carinas are of more frequent oc- 

 currence. Spirals of spines, of variable number in the different 

 series of forms, appear on the inflected, bifurcated, or intercalated 

 ribs. These spirals are more numerous on the series of less remote 

 geological age. In some cases, all spines, except those of the 

 characteristic ranges on the convex portion, are wanting. The lobes 

 in forms of more remote geological age are quite concordant with 

 those of Ceratites ; in those geologically less ancient, Professors 

 Quenstedt and Laube have first noticed the digitiform notches ex- 

 tending over the saddles, and the denticulation of the lobes in- 

 creasing in depth. 



Heraclites, Mojs. — A limited series of forms in the Norian strata 

 of the Juvavian Province, with intermediate forms, connecting 

 Her. PoescMi, Hauer, with Her. robustus, Hauer. Body-chamber 

 very short, about half of a whorl. Strong, in some cases inflated, 

 ribs on the sides. Convex portion flattened, traversed by two 

 delicate, filiform, spiral lines, sometimes with nodules at the points 

 where they run across the ribs. Her. robustus loses every trace of 

 sculpture on the convex portion as it advances in age. Sculpture 

 is likewise wanting in some forms of less remote geological age, as 

 Her. foliaceus, Dtm. The lobes are characterized by a few irregular 

 deep sections, hanging far downwards — here Her. quadrangulus, 

 Hauer. 



Sagenites, Mojs. — The known forms, belonging to this genus, are — 

 Sag. reticidatus, Hauer, Sag. Giebeli, Hauer, and Sag. inermis, Hauer. 

 In the typical forms the sculpture passes without interruption over 

 the vaulted convex portion, which sinks gradually into the lateral 

 portion ; in some aberrant forms there is an interruption in the 

 centre of the convex portion (as in Ceratites) and the narrow, 

 canaliform, unsculptured band is accompanied by nodular incrassa- 

 tions. The numerous, delicate, pliciform, transversal ribs are crossed 

 by a system of more or less undulated spiral lines, lying very close 

 to each other. Umbilicus narrow. Occasionally the shell is orna- 

 mented here and there with broad obtuse knobs. Body-chamber half 

 to three-quarters of the whorl in length. Type of the lobes aberrant 



DECADE II. — VOL. VI. — NO. X. 30 



