256 REPORT OF NATIONAL MUSEUM, 1892. 



in the slime of marshes and shallow waters or by receiving a covering 

 of such sediments as river floods usually carry. 



A few of the living Batrachia pass their whole lives in the water, but 

 the greater proportion of them are, in their adult state, air-breathing 

 paliistral animals. A smaller proportion of them live upon dry land, 

 but these, like all the others, have aqueous respiration during their 

 larval condition. Besides this, as a rule, those which are strictly land 

 animals in their adult state seek the water at the breeding season. 

 Therefore, a larger proportion of batraehian remains are likely to find 

 paliistral, than other sedimentary, intombment. The living Batrachia 

 do not constitute so conspicuous a class as do the other vertebrates, 

 and fossil batraehian remains are also comparatively rare, but among 

 the reasons for this rarity is doubtless the fact that few of the class 

 inhabit the larger bodies of water, such as those in which the more im- 

 portant formations were deposited. 



Because all fishes are of aquatic habitat the intombment of their 

 remains in the sediments of the waters in which they lived is more a 

 matter of course than it is in the case of any of the other vertebrates. 

 It is true, however, that fossil fish remains are, as a rule, less abundant 

 in the sedimentary formations than might be expected in view of the 

 comparative abundance of fishes in now existing waters. This is diffi- 

 cult to explain, even if it were now necessary to do so, but it is perhaps 

 due in part to the entire destruction of their bodies in many cases by 

 predatory enemies, in part to the large proportion of animal matter in 

 their bones, and in part to other destructive causes acting upon the 

 usually but not always fragile ichthyic skeleton. The absence of a 

 true skeleton in many fishes ought also to be taken into consideration 

 in this connection, extinct fishes of this kind being represented only, or 

 mainly, by teeth and spines. 



Mollmca. — The hard parts of mollusks, those which are preservable 

 by sedimentary intombment, consist mainly of lime carbonate with a 

 smaller proportion of animal matter than the hard parts of vertebrates 

 contain. They are sometimes internal; but these, strictly speaking, are 

 not skeletal in the sense that the bones of vertebrates are so. Usually 

 they form a protective shell which envelopes the whole, or the greater 

 part, of the animal. Much the greater proportion of the members of 

 this branch of the animal kingdom have aqueous respiration, and these 

 consequently live only in the water. The others are air-breathers and 

 live either upon the land or at the water's edge. Many species and 

 genera and some whole families of both aquatic and laud mollusks have 

 no hard parts, and their bodies are therefore immediately and com- 

 pletely decomposed after death, leaving no trace of their former exist 

 ence. The hard parts of aquatic mollusks find speedy intombment 

 in the sediments at the bottom of the waters in which they lived, while 

 those of land mollusks are liable to complete atmospheric decomposi- 



