406 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 29, 



The zone of Ammonites obtusus probably attains a thickness of from 

 80 to 100 feet ; but its actual measurement would be a matter of 

 difficulty, from the manner in which the marls have covered over the 

 bands of limestone : hence the imperfection of our estimate. 



In the lower slaty marls are numerous compressed Ammonites 

 Birchii, which fall to pieces when removed from the matrix. Higher 

 up (JSTo. 17) the same species is found in fine preservation, with Am- 

 monites obtusus. Here the shells are replaced, and the septa filled, 

 with crystallized carbonate of lime. These beautiful specimens are 

 the " Tortoise-ammonites " of the local collectors. About 40 or 

 50 feet above these beds is an irregular band of limestone (5), pro- 

 jecting from the cliff, containing nodules with very large specimens 

 of Ammonites obtusus, Sow., Am. stellaris, Sow., and Am. Broolcii, 

 Sow. Most of the nodules have a septarian structure, the veins of 

 spar intersecting and distorting the shape of the Ammonite. 



Below the Ammonitiferous nodides (5 of the section) other bands 

 of clay and marl (6 to 14) succeed. In one of these (9) are thin 

 layers of Crinoidal limestone, on the surface of which magnificent 

 specimens of Ecctracrinus Briar eus, Mill., are found, with their plant- 

 like arms laid out in all directions, and generally coated with sul- 

 phur et of iron. 



Fossils of the Zone of Ammonites obtusus. 



Ammonites obtusus, Sow. 



Brookii, Sow. 



stellaris, Sow. 



planicosta, Sow. 



Dudressieri, d' Orb. 



Ammonites Sniithii, Sow. 

 Nautilus striatus, Sow. 

 Belemnites acutus, Mill. 

 Pleurotomaria Anglica, Sow. 

 Extracrinus Briareus, Mill. 



5. The Zone of Ammonites oxtnottjs. 



Synonyms. — " Oxynoten-Schichte," Fraas, Wiirttemb. naturw. 

 Jahreshefte, 1847, p. 206. " Oxynotenlager," Quenstedt, Der Jura, 

 p. 293, 1858. " Die Schichten des Ammonites oxynotus," Oppel, 

 Die Juraformation, p. 54, 1856. " Oxynotus-bed," Wright, Quart. 

 Journ. Geol. Soc. vol. xiv. p. 25, 1858. 



This zone consists of beds of dark clays, often containing much 

 sulphuret of iron, or iron in the state of peroxide, all the fossils 

 found in the clay being either highly pyritic, or charged with per- 

 oxide of iron. The bed was exposed in the cuttings of the Bristol and 

 Birmingham, and Great Western Bailways, at Lansdown near Chel- 

 tenham, and likewise in excavating the new docks at Gloucester ; 

 and I have collected its characteristic fossils at other localities in the 

 Yale of Gloucester. 



In Dorsetshire a variety of Ammonites oxynotus, Quenst., is found 

 in a thin bed of dark pyritic marl between Charmouth and Lyme 

 Regis, near Black Yenn. It is here collected with other species, 

 which properly belong to a higher bed : the falling down of the marl, 

 by the decay of the bank, makes it difficult to separate the beds. 



At Bobin Hood's Bay, on the Yorkshire coast, the relative position 

 of this zone to the beds with Ammonites obtusus, Sow., below, and 



