All the remaining Cambrian fossils which demand any notice here are members of one or other division of the great class of the Mollusca, or “Shell-fish” properly so called.  In the Lower Cambrian Rocks the Lamp-shells (Brachiopoda) are the principal or sole representatives of the class, and appear chiefly in three interesting and important types—­namely, Lingulella, Discina, and Obolella.  Of these the last (fig. 32, i) is highly characteristic of these ancient deposits; whilst Discina is one of those remarkable persistent types which, commencing at this early period, has continued to be represented by varying forms through all the intervening geological formations up to the present day. Lingulella (fig. 32, c), again, is closely allied to the existing “Goose-bill” Lamp-shell (Lingula anatina), and thus presents us with another example of an extremely long-lived type.  The Lingulelloe and their successors; the Linguloe, are singular in possessing a shell which is of a horny texture, and contains but a small proportion of calcareous matter.  In the Upper Cambrian Rocks, the Lingulelloe become much more abundant, the broad satchel-shaped species known as L.  Davisii (fig. 32, e) being so abundant that one of the great divisions of the Cambrian is termed the “Lingula Flags.”  Here, also, we meet for the first time with examples of the genus Orthis (fig. 32, f, k, l) a characteristic Palaeozoic type of the Brachiopods, which is destined to undergo a vast extension in later ages.

[Illustration:  Fig 32.—­Cambrian Fossils:  a, Protospongia fenestrata, Menevian Group; b, Arenicolites didymus, Longmynd Group; c, Lingulella ferruginea, Longmynd and Menevian, enlarged; d, Hymenocaris vermicauda, Lingula Flags; e, Lingulella Davisii, Lingula Flags; f, Orthis lenticularis, Lingula Flags; g, Theca Davidii, Tremadoc Slates; h, Modiolopsis Solvensis, Tremadoc Slates; i, Obolela sagittalis, interior of valve, Menevian; j, Exterior of the same; k, Orthis Hicksii, Menevian; l, Cast of the same; m, Olenus micrurus, Lingula Flags. (Alter Salter, Hicks, and Davidson.)]

Of the higher groups of the Mollusca the record is as yet but scanty.  In the Lower Cambrian, we have but the thin, fragile, dagger-shaped shells of the free-swimming oceanic Molluscs or “Winged-snails” (Pteropoda), of which the most characteristic is the genus Theca (fig. 32, g).  In the Upper Cambrian, in addition to these, we have a few Univalves (Gasteropoda), and, thanks to the researches of Dr Hicks, quite a small assemblage of Bivalves (Lamellibranchiata), though these are mostly of no great dimensions (fig. 32, h).  Of the chambered Cephalopoda (Cuttle-fishes and their allies), we have but few traces; and these wholly confined to the higher beds of the formation.  We meet, however, with examples of the wonderful genus Orthoceras, with its straight, partitioned shell, which we shall find in an immense variety of forms in the Silurian rocks.  Lastly, it is worthy of note that the lowest of all the groups of the Mollusca—­namely, that of the Sea-mats, Sea-mosses, and Lace-corals (Polyzoa)—­is only doubtfully known to have any representatives in the Cambrian, though undergoing a large and varied development in the Silurian deposits.