CAMBRIAN INVERTEBRATES 



123 



aquatic invertebrate groups and their high degree of special- 

 ization in Early Cambrian times, which makes it necessary to 

 look for their origin far back in the pre-Cambrian ages; and, 

 second, the extraordinary persistence of type, not only among 

 the lamp-shells (brachiopods) but among members of all the 

 invertebrate phyla from the mid- Cambrian to the present 



Tercbratu la 



Devon -Rece nt 



Fig. 21. Brachiopods. Cambrian axd Recent. 



Lingulella (Lingula) acuminata, a fossil form ranging from Cambrian to Ordovician, 

 and the verj- similar existing form, Lingula anatina, which shows that the genus has 

 persisted from Cambrian times down to the present day. 



Lingulella (^fossil), Cambrian to Ordovician, contrasted with a living specimen of the 

 wideh- differing Tcrchratiihi, which ranges from Devonian to recent times. 



time, so that sea forms with an antiquity estimated at twenty- 

 five million years can be placed side by side with existing sea 

 forms with very obvious similarities of function and structure, 

 as in the series arranged for these lectures by Mr. Roy W. 

 Miner, of the American Museum of Natural History (Figs. 21, 

 22, 24-27). 



Except for the trilobites, the existence of Crustacea in 

 Cambrian times was unknown until the discovery of the prim- 



