344 
THE GEOLOGIST. 
is expressed by saying there is but one locality for the fossil, and bii 
one fossil for the locality. Many hundreds of specimens have bee 
found, and but two instances are known to have occurred in whic 
the head was attached to the thorax. On splitting a stone, and 
thereby disclosing one of these trilobites, except in the two cases 
just named, the head is not visible ; or what is much more frequently 
the case, one half the stone is found to contain the thorax and tail 
united, and the impression of the head, whilst in the other half are 
found the head and the impression of the body, and always in such a 
way as to show that the head had been severed from the body^ 
removed a short distance from it, as if drawn or pushed forward, 
and inverted. In the cases where the head has not been visible, it 
has generally happened that it has been concealed by a mere film of 
the imbedding matrix, and can be found with a little care. Whe 
so found it tells the same story. There are never any indications o 
eyes ; not unfrequently the tail appears somew^hat truncated, as if i 
terminal margin were slightly folded or tucked under. It is cle" 
that an inversion of the head might have been effected by a semi 
rotation either at right angles to the axis of the tody or in the direc- 
tion of that axis ; but as the anterior margin of the head is always 
found nearest the thorax, it is clear that the motion must have been 
of the latter kind. The rock in which the fossils occur has been 
pronounced by Mr. Sorby and others to be a volcanic ash, and this 
without reference to, or knowledge of, any speculations respecting 
the facts connected Avith the trilobites. Knowles Hill, on the flanks 
of w^hich they occur, is a mass of greenstone, and is marked as such 
in the map published by the Geological Survey. 
According to Burmeister, it is probable " that these animals (trilo- 
bites) moved only by swimming ; that they swam close beneath the 
surface in an inverted position, the belly upwards, and the back' 
downwards, that they made use of their power of rolling them- 
selves into a ball as a defence against attacks from above ; and that 
they lived gregariously in vast numbers, chiefly of one species."* 
The facts connected with this fossil seem capable of explanation 
by supposing that a shower of volcanic ashes, falling into the ancient 
Devonian sea in the Newton area, alarmed a shoal of these trilobites 
just then swimming by, and thereby caused them instinctively to roll 
themselves up for defence ; that the continuation of the shower, and 
possibly the presence of noxious gases, killed the unfortunate crusta- 
ceans in the rolled-up posture ; that their centre of gravity was so 
situated as to cause them to sink to the bottom on their backs ; that 
they were inhumed in the heap of ashes, which, by accumulating 
very rapidly in great quantity, produced a pressure sufl&cient to 
flatten the body, and, with very few and slight exceptions, the tail 
also ; to dislocate the head (the line of union of the head and body 
being the line of least resistance), and, after the manner in which 
slaty cleavage in rocks is probably produced, to thrust it some- 
* Burmeister's " Trilobites," Roy. Soc. p. 52. 
