128 FIELD GEOLOGY. 



Thus, in the Vertebrata, the remains of mammals and 

 birds are among the rarest of geological relics, even in 

 beds of littoral or terrestrial origin, save those of very 

 recent date ; on the other hand, marine reptiles and 

 fishes are found in almost all the later fossiliferous 

 deposits, their vertebrae, scales, and teeth being occasion- 

 ally very abundant ; they seldom occur, however, in 

 anything like a perfect condition, except in certain 

 strata which have been formed quietly and rapidly 

 enough to envelop them before dismemberment. 



Among the Invertebrates the group that is by far the 

 most useful and important to geologists is that of the 

 Mollusca ; they usually present us with a complete 

 enveloping shell or test, instead of the fragmentary 

 remains which are generally left to represent the indi- 

 viduals of the Vertebrate classes ; and from their con- 

 stant occurrence in almost every fossiliferous bed they 

 form an excellent standard of reference in considering 

 questions of relative age and position. William Smith 

 has happily likened fossil organisms to coins and an- 

 tiquities, and the remains of rnollusca are to geologists 

 as the numismatic relics of a kingdom which has lasted 

 from the earliest dawn of life upon the earth down to 

 the present day. 



Corals and Echinoderms occasionally occur in con- 

 siderable profusion, and sometimes form great thick- 

 nesses of rock. Eoraminifera too are of very general 

 distribution, but from their small size and low place in 

 the animal scale, they are not of much practical use to 

 the geologist. Crustacea are of more limited occurrence, 

 most abundant in beds of littoral origin, but rare in 

 sandy strata, and often absent in deep-sea deposits. 



