44 



POPULAR CONCHOLOGY. 



Family 2. — AMMO NIT A CEA. 



The shells composing this family have very much the 

 same structure as the Nautilacea, but the aperture has 

 processes, and the partitions are lobed or foliated; the 

 siphuncle is dorsal. — All fossils. 



Ammonites. Breyn. — Shell discoid, spiral : whorls 

 contiguous, and often all apparent ; inner sur- 

 face (the exterior covering has generally dis- 

 appeared) marked by the sinuous partitions, 

 which are transverse and deeply lobed or 

 cut; siphuncle either on the margin or to- 

 wards the back : last chamber occupying 

 more than half, or the whole of the last 

 whorl of the spire. — Between 500 and 600 species, fossil. 



These shells are all fossil, and are from half an inch to 

 two feet or more in diameter. Dr. Buckland speaks of 

 some found in the chalk near Margate which exceeded 

 four feet in diameter. They occur in many parts of the 

 world. Dr. Gerard found them in the Himalaya Moun- 

 tains, at an elevation of 1 6,000 feet : M. Menard met with 

 them in the Alps. Their numbers must have been very 

 great in former ages : M. Dufresne informed Lamarck that 

 the road from Auxerre to Avalon, in Burgundy, was abso- 

 lutely paved with them ; and it is no uncommon thing to 

 find them used in some parts of Oxfordshire, Somersetshire, 

 Gloucestershire, and Yorkshire, to pave the roads. They 

 are commonly called snake-stones, and are supposed by the 

 unlearned to be petrified snakes. In some the shell seems 

 to have been as thin as that of the Paper Nautilus. 

 Sowerby describes and illustrates in his u Mineral Con- 

 chology " many very interesting species, some in so perfect 

 a state that they preserve their bright iridescent colours. 



