154 THE 8TOEY OP THE EARTH AND MAN. 



ferous, those with one valve very convex and the 

 other very concave and anchored in the mud by long 

 spines instead of a peduncle attached to stones and 

 rocks.* There are many beautiful shells allied to 

 modern scallops,, and not a few sea-snails of various 

 soi ts. The grand Orthoceratites of the Silurian di- 

 minish in size preparatory to their disappearance in 

 the Permian, and the more modern type of Nautilus 

 and its allies becomes prevalent. Among the Crus- 

 taceans we may notice the appearance of the Limulus, 

 or king-crab, of which the single little species de- 

 scribed by Woodward from the Upper Silurian may 

 be regarded as merely a prophecy. It is curious that 

 the Carboniferous king-crabs are very small, appa- 

 rently another case of a new form appearing in 

 humble guise; but as the young of modern king- 

 crabs haunt creeks and swampy flats, while the adults 

 live in the sea, it may be that only the young of the 

 Carboniferous species are yet known to us, the speci- 

 mens found being mostly in beds likely to be fre- 

 quented by the young rather than by the full-grown 

 individuals. 



The old order of the Trilobites, which has accom- 

 panied us from Primordial times, here fails us, and a 

 few depauperated species alone remain, the sole sur- 

 vivors of their ancient race small, unornamented., and 

 feeble representatives of a once numerous and influen- 

 tial tribe. How strange that a group of creatures so 

 numerous and apparently so well adapted to conditions 

 * The Productidse. 



