Ch. XXL] 



FOSSILS OF THE LIAS. 



419 



Fig. 445. 



Ammonites siriatulus, Sow. 

 I nat. size. Upper Lias. 



Fig. 446. 

 a o 



Ammonites margaritatus, Montf. 

 Syn. A jStokesii, Sow. Middle Lias. 



Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites 

 are conspicuous. (See fig. 449.) Of Ophioderma Egertoni (fig. 

 450), referable to the Ophiuridce of Muller, perfect specimens have 

 been met with in the Middle Lias beds of Dorset and Yorkshire. 



Allusion has already been made, p. 416, to numerous zones in the 

 Lias characterized by distinct Ammonites. Two of these occur near 

 the base of the Lower Lias, having a united thickness, varying from 

 40 to 80 feet. The upper and larger of these is characterized, espe- 

 cially in the southwest of England, by Ammonites Bucklandi, and the 

 lower by Ammonites Planorhis (figs. 447, 448).* 



Fig. 447. 



Ammonites Buc7clandi, Sow. 

 " bisulcatus, Brug. 



I diam. of original. 

 a. Side view. 



&. Front view, showing month and bisnlcated keel. 



Characteristic of the lower part of the Lias of 



England and the Continent. 



Fig. 448. 



A. PZanorbis, Sow. 



| diam. of original. 



From the base of the Lower Lias 



of England and the Continent. 



These two shells have a wide range on the Continent of Europe, 

 occurring in beds which occupy similar positions in the Liassic series. 



The Extracrinus JBriareus (removed by Major Austin from Penta- 

 crinus on account of generic differences) occurs in tangled masses, 

 forming thin beds of considerable extent, in the Lower Lias of Dor- 

 set, Gloucestershire, and Yorkshire. The remains are often highly 

 charged with pyrites. This Crinoid, with its innumerable tentacular 



Quart. Journ., voL xvi. p. 376. 



