419 
Geology of the Environs of Petersburg . 
flap tail, which is often found preserved when the rest of the 
animal has perished: the shape of this tail is triangular, with a 
strong ridge or continuation of the back running down the middle; 
it is often protected by a thick shell, still remaining, under which 
the surface of the cast of the animal itself appears furrowed like 
the skin of the human thumb. The tail, when found alone, 
somewhat resembles a butterfly in shape. 
The other species, which has been named trilohites capitatus , is 
much rarer ; and has only been found as yet in the eastern part of 
the district. It differs from trilobites caudatus in having no tail, 
the plates being continued to the very extremity. In this it 
resembles some of those found in the grauwacke of South Wales 
and limestone of Dudley. But its most remarkable distinction is 
its head, which bears a remarkable face or figure. These figures 
were at first found without the body, as were the tails of the other 
species, and exercised in vain the ingenuity of naturalists, until 
they were at last luckily discovered attached to their respective 
animals. 
Both these species appear to have possessed wonderful powers of 
flexibility, and are occasionally found rolled up with their tails in 
their mouths, and disposed in other singular attitudes. Their 
colour is usually brown. 
I have found also some small terebratulites, and helicites, in the 
limestone of the Colomenca ravine. The latter are rather un- 
common. 
These argillaceous beds contain also balls of an unknown origin. 
They are round, oval or lemon shaped, with a knob at one or at 
each end. Their interior structure consists of a mass of radiated 
calcareous spar ; and on the outside the pyramid of some of 
the crystals, which is very low, sometimes projects beyond the 
