IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 59 



and completely preserved, though many of them are perfectly 

 capable of being fossilised. Almost all such remains, how- 

 ever, as we have of these three great classes, are the remains 

 of isolated individuals, which may have been accidentally 

 drowned ; or else they occur in hollow trees, or in fragments 

 of ancient soils, or in vegetable accumulations such as coal 

 and peat. There is, however, a considerable number of 

 aquatic insects (but exclusively in fresh water), and there are 

 many insects the larvae of which inhabit water, whether this 

 be fresh or salt ; so that instances of these occurring as fossils 

 are not very infrequent. 



e. Mollusca. This sub-kingdom requires little notice, since 

 the greater number of its members possess hard structures 

 / capable of being preserved in a fossil condition. Thus, the 

 horny or calcareous polypidoms of many of the Polyzoa, the 

 shells of the Brachiopods, the true Bivalves, and most of the 

 Gasteropoda, the internal skeletons of the Cuttle-fishes, and 

 the chambered shells of the Tetrabranchiate Cephalopods, all 

 occur more or less abundantly as fossils. The entire class of 

 the Tunicaries, however, presents (with one or two excep- 

 tions) no hard structures, and is hence not with certainty 

 known by any fossil representative. Amongst the Gaster- 

 opoda, again, the Sea-slugs and their allies (Nudibrancliiata) 

 possess no shell, and are unknown to the palaeontologist ; 

 whilst the shell of the Land-slugs is extremely minute, and 

 has not been certainly recognised as fossil. Lastly, the 

 air-breathing terrestrial Molluscs, from their habits, rarely 

 occur as fossils ; whilst those which inhabit rivers, ponds, 

 and lakes are less largely represented than marine forms, 

 owing to the preponderance of salt - water deposits over 

 those of fresh water. 



/. Vertebrata. The majority of Vetebrate animals possess 

 a bony skeleton, so that their preservation in a fossil state 

 so far as this point is concerned is attended with no diffi- 

 culty. Some of the fishes, however (such as the Lancelot, 

 the Hag-fishes, and the Lampreys), have no scales, and either 

 possess no " endoskeleton " or have one which is almost 

 wholly cartilaginous. The only evidence, therefore, which 

 could be obtained of the past existence of such fishes would 



