LIFE ON THE EARTH. 91 



zoic Strata, their families are pretty equally dif- 

 fused. 



Tunicata, another somewhat less abnormal group, 

 not covered by shell, is not recognized in a fossil 

 state. 



Brachiopoda abound in the Strata, and are scat- 

 tered through the modern Seas in somewhat greater 

 numbers than was formerly supposed, when dredging 

 at considerable depths was not practised. They are 

 rare in the Csenozoic Strata. Of nearly forty genera 

 and subgenera upwards of twenty appear limited to 

 the Palaeozoic Series ; only two are mentioned among 

 the Csenozoic fossils of Britain; but twelve at least 

 occur in various modern oceans. The families of 

 Spiriferidse, Orthidse, and Productidse are confined 

 to the Palaeozoic Strata; Terebratulidse, Rhyncho- 

 nellidse, Craniadse, Discinidse, and Lingulidse may be 

 regarded as of all periods ; Lingula, Discina, Crania, 

 Rhynchonella, Thecidium, Argiope, Terebratella, 

 Waldheimia, Terebratulina, Terebratula, are both 

 fossil and recent. Of these, Terebratula, Rhyncho- 

 nella, Crania, Discina, Lingula, pass through all the 

 great periods of Geology and still exist, with pecu- 

 liarities of structure and association very like those 

 which belonged to them in the earliest periods. 

 For example, the Lingulse of every age always shew 

 almost exactly equal, delicate, depressed, nearly 



