434 THE LIAS AMMONITES. 



Planites seepextinus, Be Eaan. Amm. et Goniat., p. 89, No. 20, 1825. 



Ammonites CJiciLiA, — Ibid., p. 112, No. 21, 1825. 



— MULGRAVius, Phillips. Geology of Yorkshire, p. 136, 1829. 



— FALCIFEK, Zieten. Versteiner Wiirtemb., pp. 9, 16, tabs, vii, fig. 4, 



and xii, fig. 2, 1830. 



— SEKPENTINUS, Roetiier. Versteiner. Nord-Deutsclie Oolithgeb., p. 185, 



1836. 



— — d'Orligny. Pal. Franc., Terr. Jurass., torn, i, 215, pi. 55, 



1842. 



— — Quensiedt. Flozgebirge Wiirtembergs, p. 258, 1843. 



— — Quenstedt. Die Cephalopoden, tab. vii, fig. 3, 1849. 



— — Chapuis et Bewalque. Terr, second. Luxemb., p. 68, 



pis. ix,fig. 4, and x, fig. 1, 1854. 



— — Morris. Catalogue of British Fossils, p. 295, 2nd ed., 



1854. 



— MULGRAVIUS, Siwipson. Fossils of Yorksh. Lias, p. 73, 1855. 



■ — SERPENTiNUS, Bumortier. Dep6ts Jurassiques du Bassin du Rh6ne, 



p. 50, 1874. 

 Habpoceras seepentinum, Neumayr. Zeitschr. Deutsch. geol. Gesellsch., Bd. xxviii, 



p. 909, 1875. 



— — Tate and Blake. Yorkshire Lias, p. 307, 1876. 



Diagnosis. — Shell discoidal, depressed, and carinated; whorls flat, with lateral sulcus, 

 upper border sloping towards the siphonal area, inner border abruptly truncated at the 

 umbilical margin ; ribs 100, transverse, rugose, and undulated ; curved forward in the 

 middle, and abruptly truncated at the umbilical margin ; siphonal area rounded, sloping, 

 with a subacute elevated carina in the middle, towards which the lateral ribs bend forward. 

 Suture-line tortuous, forming four lobes. Aperture with lateral processes, and a long 

 projecting abdominal spine. 



Dimensions. — Transverse diameter 180 millimetres; height of last whorl 60 milli- 

 metres ; width of ditto 30 millimetres ; width of umbilicus 80 millimetres. 



Description. — This beautiful Ammonite is a capital leading fossil for the determination 

 of the horizon in which it is found ; the shell has received so many different names that 

 it is well to master the synonymy of the species. In early life and before the ribs have 

 acquired their special form, it is readily mistaken for Harp, radians ; when it has attained 

 a diameter of forty millimetres the ribs become flexed and curved in the middle ; and they 

 retain this form until old age, when the ribs gradually disappear, and the last whorl 

 bears a smooth shell. 



An adult form of this Ammonite was well figured by Reinecke in 1818 as Argonauta 

 serpentinus, and the young form as A. Ccecilia. Sowerby, in 1821, figured an adult 

 specimen from the Upper Lias of Ilminster as Am. Strangeioaysi, and the young form 

 thereof as Am.falcifer. The following year Young and Bird figured it as A7n. Mulgravius. 



The shell is compressed, discoidal, strongly carinated, and provided with a prominent 

 carina ; the whorls are flattened and slightly convex ; their sides covered with numerous 



