GASTEROPODA.
Fossil univalves—the remains of spiral and limpet-like shells—are not wanting in any but the very oldest fossili ferous rocks (" lingula flags"). From the lower Silurian, where less than 100 species, referable to scarcely more than ten genera, are found, they increase in number and variety slowly and regularly up to the newer tertiaries, which have afforded ten times as many genera and twenty times as many species. The total number of fossil marine univalves is less than 6000 ; the recent exceed 8000 ; and although we may expect to discover more new fossil species than recent, yet it 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 Pulmonifera, 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 ih 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.
The earlier describers of fossil univalves unhesitatingly recognised many familiar recent genera, even in the older rocks. But their Melanice were marine shells ; the sup Paleozoic Univalves.
The more closely paheozoic univalves are examined, so much the more do they appear to differ from ordinary recent types ; and the search for allied forms has to be conducted amongst the rare and minute and least understood of recent shells.