430 



THE LIAS AMMONITES. 



Family.— A^GOCERkTlDM— continued. 



Genus Yl — Harpoceras, TFaa^en, IS69. 



Family — Falciferi, 



Von Buck, 1832. 

 — (TOrbif/ny, 1841. 



Falciferek, Quenstedt, 1849. 



Family — Disci sive clypeifoemes, Giehel, 1852. 

 Falciferi, Giehel, 1852. 



— Seebach, 1864. 



Fig. 194. — Harpoceras ser- 

 pentinum, lieinecke. 



The Falciferi of Von Buck have long been recognised as a remarkable group of Am- 

 monites, which have undergone several changes of nomenclature, still all authors agree on 

 the special characters on which they were originally described and figured for the first time 

 in the ' Ueber Ammoniten.' The external outhne of their shells is of various forms, the 



ventral area is always keeled with a round or angular border ; 



the sides of the whorls are ornamented vi^ith elegant falciform ribs 

 of various degrees of thickness. The aperture of the shell is in 

 general sickle-shaped, with lateral auricles and a long, pointed 

 ventral process, as seen in one of the typical forms, Harpoceras 

 serpentinum^ Reinecke (fig. 194). The body-chamber is from one 

 half to two thirds the length of the whorl. The suture-line is mode- 

 rately ramified. The siphonal lobe is in general short and bifurcate, 

 with divergent branches, and always shorter than the principal 

 lateral lobe, which is usually largely ramified ; the inner lateral 

 lobe is much smaller than the principal, and succeeded by several still smaller auxiliaries. 

 The Aptychis is thin calcareous and bivalved, with a shelly coating and several longi- 

 tudinal folds, as seen in Harpoceras Levisoni (PI. LXI, fig. 4). 



When the shell is preserved the radii on the sides of the whorls in many species are 

 in the highest degree fine and delicate, as in Harpoceras opalinum. It is the 

 external lamina of the shell which fills in all the unevenness between the thickness and 

 divisions of the folds of the inner lamina. All these folds have a falciform figure ; they 

 first bend forward, then incline backward, and near the siphonal area again turn forward 

 towards the aperture, so that a well-formed, strongly-bent sickle is described thereby. 

 The siphonal area in general forms a sharp ridge, which sometimes contains the siphuncle 

 and gives a marked character to the shell. The umbilicus is usually well open, although 

 in a few species it is contracted. The Falciferi are very rich in species, which well 

 characterise the horizons in which they are found. 



The oldest forms are found in the Middle Lias. They are very numerous in the 

 Upper Lias, which is the metropolis of the species ; they are found in the Lower and 

 Middle Jurassic rocks, finally disappearing in the Kimmeridge Clay with Harpoceras 

 Zio, Oppel. 



