824 GEOLOGY OF THE BRISTOL COAL-FIELD. 



noticeable to anyone passing, and therefore, form a capital guide to 

 this wonderful deposit of Ichthyodorulites. The defensive spines of 

 Ctenacanthus sometimes attain a length of 20 inches or two feet. 

 These formidal weapons formed part of the dorsal fins of immense 

 sharks, with grinding teeth like those found near Port Jackson, in 

 Australia. In consequence of these fishes being cartilaginous in 

 their structure, only these spines and the teeth have been preserved, 

 the soft portions having probably disappeared. 



The following is a list of fossil contents : — 



Cladodus conicus (Ag.), Milleri (Ag,), mirahilis (Ag.); 

 Beltoptychius acutus (Ag.); Tomodus convexus (Ag.); Chomatodm 

 ductus (Ag.), linearis (Ag ) ; Cochliodus contortus (Ag.) ; 

 Ctenaca7ithus brevis (Ag.), major {Ag.), tenuistrtatus {Ag ) ; Helodus 

 gilherulus (Ag.), loevissimus (Ag,), suhteres (Ag.), turgidus (Ag.) ; 

 Onchus hamatus (Ag.), sulcatus (Ag.); Oracanthus Milleri (Ag.), 

 minor (Ag.), pustulosus (Ag.); Orodus cinctus (Ag.), ramosus (Ag.); 

 Fsammodus porosus (Ag.), ru^osus (Ag.). 



A long series of interesting beds follow to the end of the quarry, 

 and terminate in a deep ravine leading to the summit of Durdham 

 Down, near what is called the Sea Wall, from which a magnificent 

 panorama is beheld, comprising the fine anchorage ground of 

 Kingroad, backed up by the Welsh Hills. 



It is on the side of this ravine that a botanical rarity may be 

 gathered, Grimmia orbicularis, a round fruited moss. 



On passing the opening of this ravine, we come to a singular 

 series of beds, 167 feet in thickness, of Oolitic limestone, so full of 

 false joints as to give the beds the appearance of having a vertical 

 position, although they really dip 30°. The oolitic structure is 

 extremely fine, each granule having in its centre a minute speck of 

 sand, A great number of microscopical examinations have been 

 made, but have hitherto failed in finding any organic nucleus. 

 On passing these, we come to more regular beds, containing fossils, 

 but only stay to notice one (16). — [1433], having a brown colour, 

 situated 44 feet above the oolitic strata just mentioned, and is 

 entirely composed of myriads of the valve s of Terebratula hastatttf 

 and 6 inches thick. 



