88 HISTORICAL PALZONTOLOGY. 
lived type. The Zzngulelle and their successors, the Lizgula, 
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 Zzmgulel/l@ become much more 
abundant, the broad satchel- shaped species known as JZ. 
Davisii (fig. 32, ¢) 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, &, 2) a characteristic Paleozoic type of 
Fig. 32.—Cambrian Fossils : a, Protospongia fenestrata, Menevian Group; 6, Avrenz- 
colites didymus, Longmynd Group; c, Lingulella ferruginea, Longmynd and Meneyian, 
enlarged ; d, Hymenocaris vermicauda, Lingula Flags; e, Lingulella Davisii, Lingula 
Flags; 4, Orthis lenticularis, Lingula Flags; g, Theca Davidiz, Tremadoc Slates; %, 
Modiolopsis Solvensis, Vremadoc Slates; 7, Obolella sagittalis, interior of valve, Mene- 
vian; 7, Exterior of the same; 4, Orthis Hicksit, Menevian; 2, Cast of the same; mm, 
Olenus micrurus, Lingula Flags. (After Salter, Hicks, and Davidson.) 
the Brachiopods, which is destined to undergo a vast extension 
in later ages. 
Of the higher groups of the A7Zo//usca 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” (P¢eropoda), of which the most 
characteristic is the genus Zheca (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 (Lamedlibranchiata), 
though these are mostly of no great dimensions (fig. 32, 7). 
Of the chambered Cephalopoda (Cuttle-fishes and their allies), 
