344 



THE GEOLOGIST. 



is expressed by saying there is but one locality for the fossil, and but 

 one fossil for the locality. Many hundreds of specimens have been 

 found, and but two instances are known to have occurred in which 

 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 

 wav 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. When 

 so found it tells the same story. There are never any indications of 

 eyes ; not unfrequently the tail appears somewhat truncated, as if its 

 terminal margin were slightly folded or tucked under. It is clear 

 that an inversion of the head might have been effected by a semi- 

 rotation either at right angles to the axis of the body 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 with the trilobites. Knowles Hill, on the flanks 

 of which 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 

 1 1 ley were inhumed in the heap of ashes, which, by accumulating 

 very rapidly in great quantity, produced a pressure sufficient to 

 Batten 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," Boy. Soc. p. 52. 



