THE PRIMOKDIAL, OR CAMBRIAN AGE. 41 



resemblances to groups of creatures, in more modem 

 times quite distinct from each otlier. He lias also 

 found that the modern Lmgulae are very tenacious of 

 life, and capable of suiting themselves to different 

 circumstances, a fact which, perhaps, has some con- 

 nection with their long persistence in geological time. 

 They are in any case members of the group of lamp- 

 shells, creatures specially numerous and important in 

 the earlier geological ages. 



The Lingulae are especially interesting as ex- 

 amples of a type of beings continued almost from the 

 dawn of life until now ; for their shells, as they exist 

 in the Primordial, are scarcely distinguishable from 

 those of members of the genus which still live. While 

 other tribes of animals have run through a great 

 number of diflferent forms, these little creatures re- 

 main the same. Another interesting point is a most 

 curious chemical relation of the Lingula, with refe- 

 rence to the material of its shell. The shells of mol- 

 lusks generally, and even of the ordinary lamp-shells, 

 are hardened by common limestone or carbonate of 

 lime: the rarer substance, phosphate of lime, is in 

 general restricted to the formation of the bones of 

 the higher animals. In the case of the latter, this 

 relation depends apparently on the fact that the 

 albuminous substances on which animals are chiefly 

 nourished require for their formation the presence 

 of phosphates in the plant. Hence the animal 

 naturally obtains phosphate of lime or bone-earth 

 with its food, and its system is related to this chemi- 



