1/8 HISTORICAL PALEONTOLOGY. 



peculiar in consisting each of three, four, five, or more rows of 

 large imperforate plates, whilst there are sometimes four or ten 

 rows of plates in the " ambulacral areas " also : so that there 

 are many more than twenty rows of plates in the entire shell. 

 Some of the Palaeozoic Sea-urchins, also, exhibit a very pecu- 

 liar singularity of structure which is only known to exist in a 

 very few recently-discovered modern forms (viz., Calveria and 

 Phormosoma). The plates of the inter - ambulacral areas, 

 namely, overlap one another in an imbricating manner, so as 

 to communicate a certain amount of flexibility to the shell ; 

 whereas in the ordinary living forms these plates are firmly 

 articulated together by their edges, and the shell forms a rigid 

 immovable box. The Carboniferous Sea-urchins which ex- 

 hibit this extraordinary peculiarity belong to the genera Lepi- 

 dechinus and Lepidesthes, and it seems tolerably certain that 

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

 the much more abundant genus Arch&oddaris. The Carbon- 

 iferous Sea-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 Melonitcs, 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 (Microconckus) Carbonarius, of the natural size, attached to a fossil 

 plant, and magnified. Carboniferous .Britain and North America. (After Davvson.) 



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, 



