[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. [Illustration: 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.)] 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:--