70 PALEONTOLOGY 



is evident that, in comparison with past conditions, the group 

 of univalves has only now attained its maximum development, 



Between the extinct and living air-breathers the numerical 

 discrepancy is still greater. About 300 land-snails, and half 

 as many fresh-water Pvlmonifera, are enumerated in the fossil 

 catalogues ; but the greater part of these are recent species, 

 and the whole bears no proportion to the number of living 

 land-snails, which exceeds 4000. That many more have 

 formerly existed is indicated by the fact, that the fossil land- 

 snails of the older tertiaries of Europe are entirely different 

 from their living successors, and most nearly represented at 

 the present time in the West Indies and Brazil. The generic 

 forms peculiar to oceanic islands (remains of old continents) 

 are more numerous than those of the mainlands, as if this 

 order had once been more important. But the circumstances 

 favourable to their petrifaction must have been of such rare 

 occurrence as to preclude the probability of attaining more 

 than the scantiest information concerning them. 



From the large and proportional number of living Gas- 

 teropods, and the great amount of information which has been 

 obtained of late years respecting their structure and habits, it 

 might be expected that the affinities of the fossil univalves 

 would be easily worked out, and their indications fully 

 interpreted. Such, however, is not the case. Univalve shells 

 present no internal markings, easily accessible like those of 

 bivalves, and exhibiting the essential characters of the soft 

 parts ; and their external forms are often so overlaid with 

 ornament, and disguised by mimetic characters, as to mislead 

 upon a first examination. Shells of any family may be 

 limpet-shaped, or turreted, or discoidal, plain or ornamented. 

 It is more desirable to ascertain whether they have been 

 nacreous or porcellanous ; whether the apex (or nucleus) 

 presents any peculiarities; and if operculated, whether the 

 operculum was pauci- or multi-spiral 



