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.) a characteristic Palæozoic type of the Brachiopods, which is destined to undergo a vast extension in later ages.
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
Fig. 33.—Fragment of Dictyonema sociale, considerably enlarged, showing the horny branches, with their connecting cross-bars, and with a row of cells on each side. (Original.) 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.
An exception, however, may with much probability be made to this statement in favour of the singular genus Dictyonema (fig. 33), which is highly characteristic of the highest Cambrian beds (Tremadoc Slates). This curious fossil occurs in the form of fan-like or funnel-shaped expansions, composed of slightly-diverging horny branches, which are united in a net-like manner by numerous delicate cross-bars, and exhibit a row of little cups or cells, in which the animals were contained, on each side. Dictyonema has generally been referred to the Graptolites; but it has a much greater affinity with the plant-like Sea-firs (Sertularians) or the Sea-mosses (Polyzoa), and the balance of evidence is perhaps in favour of placing it with the latter.
LITERATURE.
The following are the more important and accessible works and memoirs which may be consulted in studying the stratigraphical and palæontological relations of the Cambrian Rocks:—
In the above list, allusion has necessarily been omitted to numerous works and memoirs on the Cambrian deposits of Sweden and Norway, Central Europe, Russia, Spain, and various parts of North America, as well as to a number of important papers on the British Cambrian strata by various well-known observers. Amongst these latter may be mentioned memoirs by Prof. Phillips, and Messrs Salter, Hicks, Belt, Plant, Homfray, Ash, Holl, &c.