EVIDENCES OF PKIMITIVE LIFE WALCOTT. 247 



sediment were deponited over them, changed into rock, elevated into 

 mountain masses, and later eroded to form the present mountains 

 and canyons. 



We have long considered that the trilobite (pi. 9) was the most 

 highly developed animal in Cambrian time, but a few summers ago ^ 

 a crustacean was found by the author's son, Sidney, in the great 

 fossil bed, that was the king of the animal world in its day (pi. 10, 

 Sidneyia inexpectans) . That it was prepared to assert its right to 

 the control of the Cambrian sea is shown by the claws with which 

 it was armed (pi. 10). 



EVOLUTION OF EARLY IMARINE LIFE. 



Marine life, as already noted, began in times long antedating our 

 oldest fossil records, but we are obliged to take up its study at the 

 beginning of Cambrian time. Several groups have been studied in 

 a preliminary way, and from these certain deductions have been 

 drawn. The first that I wdll mention is one of the most unlikely of 

 animals to be preserved in fine condition in the fossil state. 



MIDDLE CAMBRIAN HOLOTHURIANS.^ 



That the tests or shells of trilobites and merostomes should be 

 finely preserved in a fine-grained, silico-argillaceous rock is rather 

 to be expected, but, with past experiences in mind, I was not pre- 

 pared to find entire holothurians. That they are present and show 

 many details of structure (pi. 11) is most instructive and satisfac- 

 tory, since their occurrence records for the first time, with the ex- 

 ception of some scattered calcareous spicules and plates, the pres- 

 ence of this class of organisms in any geologic formation. Any 

 calcareous matter that may have been present in them was prob- 

 ably removed by solution while the animal w^as in the mud and 

 before it l^ecame fossilized. That carbonic acid was present in the 

 mud and immediately adjoining water is suggested by the very 

 perfect state of preservation of the numerous and varied forms of 

 life. These certainly would have been destroyed by the worms and 

 predatory crustaceans that w^ere associated with them if the animals 

 that dropped to the bottom on the mud or that crawled or were 

 drifted onto it had not at once been killed and preserved with little 

 or no decomposition or mechanical destruction. This conclusion 

 applies to nearly all parts of a limited deposit in the fossil quarry 

 (pi. 8) about 6 feet in thickness, and especially to the lower 2 feet 

 of it. 



1 In 1910. 



2 Smithsonian Misc. Coll., vol. 57, nos. 2 and :!. 



A holothurian is defined as a sea-cucumber or similar echinoderm. 

 Medusa, a jellyfish. 



