166 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



The fossils of the lower Cambrian are not absolutely the oldest 

 known, but it is the oldest fauna which is represented with sufficient 

 completeness for a general view, and is, therefore, interesting to biologists. 



Walcott says that no plants are known in the rocks of the lower 

 Cambrian, and that he has satisfied himself, after a study of all the 

 reputed species of algae, that they are not plants, but the trails of worms 

 or molluscs. 



The number of species is small, but their diversity is most note- 

 worthy and remarkable. 



Walcott' s collection of 141 American species from the lower Cam- 

 brian is distributed over most of the marine groups of the animal 

 kingdom, and, except for the absence of the remains of vertebrates, the 

 whole province of animal life is almost as completely covered by these 

 141 species as it could be by a collection from the bottom of the modern 

 ocean. 



Four of the American species are sponges, two are hydrozoa, nine 

 are actinozoa, one an echinoderm, twenty-nine are brachiopods, three are 

 lamellibranchs, thirteen are gasteropods, fifteen are pteropods, eight are 

 Crustacea, fifty-one are trilobites, and the trails and burrows show the 

 existence of at least six species of bottom forms, probably worms or 

 Crustacea. 



The most noteworthy characteristic is the completeness with which 

 these new species outline the whole fauna of the modern sea-floor. 



Nothing brings home more vividly to the zoologist a picture of the 

 diversity of the lower Cambrian fauna and of its intimate relation to the 

 bottom fauna of to-day than the thought that he would have found, on 

 the old Cambrian shore, about the same opportunity to study the embry- 

 ology and anatomy of pteropods, gasteropods and lamellibranchs and 

 Crustacea and medusa3 that he now has at a marine laboratory, and that 

 his studies in phylogeny would have had about the same form then that 

 they have now. 



Biological evidence based on embryology and anatomy and on the 

 habits and affinities of animals is justly regarded, by zoologists at least, 

 as a more perfect record of the early history of life than palaeontology, 

 and we accept, without question, proofs of phylogeny which refer to a 

 time very much more remote than the age of the oldest fossils. 



We must not forget, however, that our generalizations in primitive 

 phylogeny rest for the most part on the study of swimming or floating 

 larvae of minute size and simple structure, which we can have little hope 

 of finding as fossils. 



