414 THE LIAS AMMONITES. 



it has four lateral branches on each side, which augment in size from without inwards, 

 the terminal branch is the largest and bifurcated. The siphonal saddle is as large as the 

 principal lateral lobe, and terminates in eight folioles, four on each side, which are nearly 

 the copies of each other. The principal lateral lobe is almost symmetrical in structure, 

 there are two short branches on each side of the stem near the base, and two large terminal 

 branches, which bifurcate and trifurcate in their terminations, and are deeply divided 

 by a small accessory saddle. The lateral saddle nearly resembles the siphonal in 

 magnitude, structure, and symmetry. The inner lateral lobe is about half the size of the 

 principal ; the stem has two short lateral processes on each side, and the summit divides 

 irregularly into three branches. The inner lateral saddle is much smaller than the 

 outer lateral saddle, and terminates in three folioles. The auxihary lobes, three in 

 number, are small and unimportant. 



Affinities and Bijferences. — This Ammonite appears to vary very little at different 

 periods of growth, preserving nearly always the same form. Some specimens, however, 

 are more convex and compressed than others ; a few have the thickest portion of the shell 

 near the umbilicus, from whence the shell is bevelled away towards the siphonal area, and 

 the body-chamber then assumes a somewhat triangular form. I have large specimens 

 of this from the Lias of Wasseralfingen, which strongly resemble the large forms obtained 

 from Frocester Hill; in other varieties, as in the type fossil, a very fine mould, 

 figured by Zieten, Verstein. Wiirtemb., tab. Ixviii, the whorls are more curved and 

 inflated than is the case in the beautiful specimen I have figured in PI. LXXIV, figs. 3 — 5, 

 in which the bevelling away of the shell between the side wall and the umbilical suture 

 is very well shown, as well as the suture-line on its sides. 



Locality and StratigrapUcal Position. — I have collected this Ammonite from the 

 hard argillaceous shales at the base of the yellow sand above the Alum Shale at Blea 

 Wyke, near Robin Hood's Bay, Yorkshire, associated with Harpoceras variahile, H. 

 insigne, and H. striatulum, and other fossils of the Jurense-zGne. In Gloucestershire I 

 know it only from the Ammonite bed of the Liassic sands at Erocester Hill. 



Foreign Distribution. — Prof. Quenstedt has shown the importance of this zone of the 

 Upper Lias in Wiirtemberg and Swabia; Prof. Leonhard in Baden; Profs. Strombeck, 

 Ewald, and Credner in North Germany in many localities. Prof. R. Lepsius has described 

 the Jurensis-Mergel in Lower Elsace ; M. Terquem near Metz, Moselle ; M, H. Harte near 

 Bayeux. In addition Thouars, Deux-Sevres ; Verpilliere and St. Quentin, Isere ; Charolles, 

 Saone-et-Loire ; Mont d'Or, near Lyons, Saint-Romain, St. Eortunat,Poleymieux, Rhone; 

 Semur, Cote d'Or ; Saint-Julien, Charlieu ; and Uhrweiler-Selzbrunnen, Bas-Rhin, may 

 be cited as localities where this shell has been collected, and where the zone of Lytoceras 

 Jurense is developed. I have given the geographical distribution of this zone of life in 

 Europe somewhat in detail as it has been much misunderstood by English geologists, 

 and has not received the consideration to which it is entitled from students of Jurassic 

 geology. 



