THE CAMBRIAN PERIOD. 



to the existing "Goose-bill" Lamp-shell (Lingula anatina), and 

 thus presents us with another example of an extremely long- 

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

 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 Lingulella become much more 

 abundant, the broad satchel-shaped species known as L. 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, /, 

 k, /) a characteristic Palaeozoic 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-s\vimming 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 (Poly- 

 zoa) 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 state- 

 ment in favor of the singular genus 

 Dictyoncma (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 



Fig. 33. Fragment of 

 Dictyonema sociale, con- 

 siderably enlarged, show- 

 Ing the horny branches, 

 with their connecting 

 cross-bars, and with a row 

 of cells on each side. 

 (Original.) 



