88 



HISTORICAL PALEONTOLOGY. 



Jived type. The Lingulellce and their successors, the Lingulce, 

 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 Lingulellcz become much more 

 abundant, the broad satchel - shaped species kjjown 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, Pr&tospongiajenestratfi, Menevian Group; d, Arenz- 

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

 enlarged; d, Hymenocarisvermicauda, Lingula Flags; e, Lingulella Davisii, Lingula 

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

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

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

 O.'enus micrurtts, 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 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 (Lainellibranchiata}, 

 though these are mostly of no great dimensions (fig. 32, h). 

 Of the chambered Cephalopoda (Cuttle-fishes and their allies), 



