268 Mr. Pengelly, on Devonian Fossils. [May 25, 



kind. The rock in which the fossil occurs 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 so marked in the map published by the 

 Geological Survey. 



According to Burmeister it is probable " that these animals (tri- 

 lobites) moved only by swimming ; that they swam close beneath the 

 surface in an inverted position, the belly upwards,'the back downwards, 

 and that they made use of their power of rolling themselves into a ball 

 as a defence against attacks from above ; that they' lived gregariously in 

 vast numbers, chiefly of one species."* The facts connected with this 

 fossil, which have been described, seem capable of explanation by sup- 

 posing 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 all to sink to the bottom on their backs ; 

 that they were inhumed in the heap of ashes, which, by accumulating 

 very rapidly and in great quantity, produced a pressure sufficient to 

 flatten the body, and, with a few and very slight exceptions, the tail 

 also, to dislocate the head (the line of union of the head and thorax 

 being the line of least resistance) and, after the manner in which slaty 

 cleavage in rocks is probably produced, to thrust the head some little 

 distance in advance of the body. By such a process the head would 

 be inverted, and in such a way that the severed parts of the creature 

 would take the relative positions which have been described. 



Though with the exception of a scale of holoptychius found, 

 according to Professor Phillips, at Meadfoot, near Torquay, ichthyo- 

 lites are not recorded as occurring in the Devonian rocks of Devon 

 and Cornwall, it is nevertheless certain that fish did exist within the 

 area, at the time under consideration. The speaker exhibited a fossil 

 which he had found in the Devonian slates at Looe, in Cornwall, and 

 which Sir Philip Egerton, and other eminent paleeontologists, had pro* 

 nounced to be an ichthyodorulite, or defence-spine of a fish. 



In conclusion, Mr. Pengelly remarked, that the time to which he 

 was limited had compelled him to be more hurried in his descriptions 

 than he could have wished, to omit all mention of the echinoderms, and 

 the various classes of shells, more or less abundant amongst the fossils 

 he had been considering, and had only allowed him to strike certain 

 veins of thought which it would have been desirable to have worked 

 thoroughly. 



[W. p.] 



Burmeister's Trilobites, Ray Society, page 52. 



