182 HISTORICAL PALEONTOLOGY. 



a similar flexibility of the shell existed to a less degree in 

 the much more abundant genus Archaocidaris. The Carbon- 

 iferous S^a-urchins, like the modern ones, possessed movable 

 spines of greater or less length, articulated to the exterior of 

 the shell; and these structures are of very common occur- 

 rence in a detached condition. The most abundant genera 

 are Archaocidaris and Pal&chinus; but the characteristic 

 American forms belong principally to Melonites, Oligoporus, 

 and Lepidechinus. 



Amongst the Annelides it is only necessary to notice the little 

 spiral tubes of Spirorbis Carbonarius (fig. 120), which are 



Fig. 120. Spirorbis (Microconchus) Carbonarius, of the natural size, attached to a 

 fossil plant, and magnified. Carboniferous, Britain and North America. (After 

 Damon.) 



commonly found attached to the leaves or stems of the Coal- 

 plants. This fact shows that though the modern species of 

 Spirorbis are inhabitants of the sea, these old representatives 

 of the genus must have been capable of living in the brackish 

 waters of lagoons and estuaries. 



The Crustaceans of the Carboniferous rocks are numerous, 

 and belong partly to structural types with which we are already 

 familar, and partly to higher groups which come into existence 

 here for the first time. The gigantic Eurypterids of the Upper 

 Silurian and Devonian are but feebly represented, and make 

 their final exit here from the scene of life. Their place, how- 

 ever, is taken by peculiar forms belonging to the allied group 

 of the Xiphosura, represented at the present day by the King- 

 crabs or "Horse-shoe Crabs" (Limulus}. Characteristic forms 

 of this group appear in the Coal-measures both of Europe and 

 America; and though constituting three distinct genera (Prest- 

 wichia, Belinurus, and Euproops}, they are all nearly related 

 to one another. The best known of them, perhaps, is the 



