THE CARBONIFEROUS PERIOD. 181 



minute apertures or " pores, " through which the animal can 

 protrude the little water-tubes ("tube-feet") by which its loco- 

 motion is carried on. In the other five zones the so-called 

 " inter-ambulacral areas " the plates are of larger size, and 

 are not perforated by any apertures. In all the modern Sea- 

 urchins each of these ten zones, whether perforate or imper- 

 forate, is composed of two rows of plates; and there are thus 

 twenty rows of plates in all. In the Palaeozoic Sea-urchins, on 

 the other hand, the " ambulacral areas " are often like those 

 of recent forms, in consisting of two rows of perforated plates 

 (fig. 119); but the "inter-ambulacral areas" are always quite 

 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 



Fig. 119. Palcecfiinut ellipticus, one of the Carboniferous Sea-urchins. The left- 

 hand figure shows one of the " ambulacral areas " enlarged, exhibiting the perforated 

 plates. The right-hand figure exhibits a single plate from one of the "inter-ambu- 

 lacral areas." (After M'Coy.) 



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 



