Crania is one of the oldest living types, ranging upwards from the lower Silurian. One of the earliest species appears to have been unattached, and another to have had hinge-teeth. Crania Ignabergensis, of the chalk of Sweden, has the valves externally alike, being attached only when very young. The internal markings of C. antiqua, and other fossil species, are remarkably grotesque. Lower valves of this genus and The cicliunt are not uncommon, attached to the tests of sea-urchins, in the chalk ; but upper valves are scarce, either detached or in situ.
The Discinidcv are also ancient fossils, few in number, but appearing in every period. Some of the palwozoic Discintr (= Orbiculoidea, d'Orb.) cannot be generically distinguished from the recent species by any characters with which we are as yet acquainted ; but others (= Trematis, Sharpe) are orna mented with quincuncial punctures, and the casts exhibit indications of diverging internal plates, which imply very considerable difference in the organization of the animal. The genus Siphonotreta (Venieuil), peculiar to the Silurian forma tions, is covered with moniliform tubular spines.
Lingula, which has given its name to one of the oldest fos siliferous rocks, is another form occurring unchanged in strata of every period. Only 34 species are known, and none of them
are very common. The latest British Lingula is found in the coralline crag (older plioccne) of Suffolk : the nearest living species is as far off as the Philippines. L. Davisii, of the " lingula flags " in North Wales, has a pedicle groove in the ventral valve, by which the posterior adductor (or cardinal muscle) must have been divided into two elements, as in the genus Obolus ; externally it has all the appearance of an ordi nary existing shell. From the fragments of Lingula in the lower Silurian stiper stones of Shropshire, they appear to belong to a species distinct from L. Davisii. Obolus, Eichw. (= Ungula, Pander) is so abundant in the lower Silurian sandstones of Sweden and Russia as to have given its name to the " obolite grit." In England it occurs only in the upper Silurian of Dudley. The shell is horny in texture, and often stained blue, like the Lingula, by the presence of phosphate of iron. In shape it is regularly oval, and differs from Lingula in the character of the internal muscular impressions.