AMALTHEUS SPINATUS. 403 



Ammonites Hawskerensis, Simpson. Foss. York. Lias, p. 90, 1855. 



— SPINATUS, d'Orbigny. Pal. Fran9aise, Ter. Jurass., t. 52, p. 209, 1842. 



— cosTATUS, Quenstedt. Die Cephalopoden, pi. v, figs. 10 a, b, 1849. 



— SPINATUS, Giebel. Fauna der Vorwelt, Bd. iii, p. 537, 1852. 



— — Chapuis et Deivalque. Fossiles terr. secondaires, Mem. 



cour. I'Acad. Roy., torn, xxv, 

 p. 49, pi. vi, fig. 4, 1854. 



— — Morris. Catal. British Fossils, p. 295, 1854. 



— — Dumortier. Depots Jurassiques, partie iii, p. 213, 1869. 



— — Brauns. Untere Jura, p. 240, 1871. 



Amaltheus — Neumayr. Zeitschr. Deutsch. geol. Gesellsch., Bd. xxvii, 



p. 886, 1875. 



— — Tate and Blake. Yorkshire Lias, p. 295, 1876. 



Diagnosis. — Shell discoidal, compressed, slightly involute ; umbilicus widely open ; 

 ribs prominent, sharp, bituberculated, separated by wide valleys, straight on the sides, and 

 bent suddenly towards the aperture at an acute angle ; siphonal area wide and concave ; 

 keel prominent, crenated, with small chevroned ribs, convex anteriorly ; aperture quadran- 

 gular, outer border with two sinuses separated by the median carina, inner border slightly 

 grooved by the turn of the spire. 



Dimensions — lar^e shell. — PI. LV. Transverse diameter 180 millimetres ; width of 

 umbilicus 75 millimetres; height of aperture 60 millimetres, width 70 millimetres. 



Smaller shell.— V\. LVI, fig. 5. Diameter 100 millimetres; width of umbilicus 43 

 raiUimetres; height of aperture 34 millimetres, width 30 millimetres. 



Smallest shell. — PI. LVI, fig. 1. With test preserved showing the sharpness of the 

 spines, ribs, and carina. Diameter 53 millimetres ; width of umbilicus 22 millimetres ; 

 height of aperture 17 millimetres, width 18 millimetres. 



The measurements of the French specimen, given by Professor A. d'Orbigny, were 

 made on a shell, 135 millimetres in diameter, and the proportions in relation to the 

 diameter were — width or height of the last whorl -yqq, involution of the whorls y^ o, thick- 

 ness of the last whorl -^^^, width of umbilicus -^^q. 



Description. — This is one of the most characteristic Ammonite-forms from the upper 

 beds of the Middle Lias, always presenting its typical features and appearing as a leading 

 fossil in the horizon to which it is limited. The shell is discoidal, compressed, with a wide 

 siphonal area, having an elevated keel in the middle with concave depressions on each side ; 

 the keel is formed of short chevroned ribs, whicb produce acarinated structure with small 

 arches, the convexities of which are directed forwards ; the ribs, twenty-two to twenty-four 

 in number, are sharp and straight on the sides, and bent forward towards the aperture at 

 the angle ; they each carry two tubercles, one blunt near the umbilical space, and another 

 sharp, prominent, and thorn-like, near the siphonal angle ; these spines are very distinct 

 when the shell is preserved, as in PI. LVI, fig. 1, but are not so demonstrable on the mould ; 

 the angle formed by the bending of the rib develops another considerable prominence of 



