88 



HISTORICAL PALEONTOLOGY. 



lived type. The Lingulellce and their successors, the Litigufa, 

 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 Lingiilelltz become much more 

 abundant, the broad satchel - shaped species known as Z. 

 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, /) a characteristic Palaeozoic type of 



Fig. 32. Cambrian Fossils : a, Protospongiafenestrata, Menevian Group; b, Arettt- 

 colitesdidymus, Longmynd Group; c, LinguleUafermginea, Longmynd and Menevian, 

 enlarged; d, Hymenocaris vermicauda, Lingula Flags; e, Lingulella Davisii, Lingula 

 Flags;/, Orthis lenticularis, Lingula Flags; g, Theca Davidii, Tremadoc Slates; h, 

 Modiolopsis Solvensts, Tremadoc Slates ; i, Obolella sagittalis, interior of valve, Mene- 

 vian ; /, Exterior of the same ; k, Orthis Hicksii, Menevian ; /, Cast of the same ; m, 

 Olenus micrurus, Lingula Flags. (Alter Salter, Hicks, and Davidson.) 



the Brachiopods, which is destined to undergo a vast extension 

 in later ages. 



Of the higher groups of the Molhisca 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 " (Ptervpoda), 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), 



