ECHINOIDEA. 237 



Olif/oporus (fig. 131, B) is very similar to Melonites, but 

 the ambulacral areas consist each of only four rows of 

 plates. 



In the genus Archceocidaris, well known as a Carbon- 

 iferous type, the test is spheroidal, the ambulacra are only 

 two-rowed, and the interambulacra are wide, and are com- 

 posed of three or more rows of plates. The interambulacral 

 plates carry each a large perforated primary tubercle, and 

 have the curious character, as shown by Mr John Young, 

 that some of their edges are bevelled off, this clearly ap- 

 pearing to indicate a certain amount of flexibility in the 

 test, similar to what we have seen in the group of the 

 EcMnothuridce. 



A much nearer approach to the type of the Echino- 

 tlmridce, however, is made by some other genera of the 

 Pcrischoechinidce. In the forms in Question (Periscliodo- 

 mus, Lepidechinus, &c.) the plates of the test overlap one 

 another in an imbricating manner, as in the recent Calveria, 

 and the test must have been quite flexible. In the Echino- 

 thuridce, however, the imbrication of the interambulacral 

 plates is from above downwards, and that of the ambulacral 

 plates from below upwards, exactly the reverse of this 

 arrangement taking place among the flexible Perischoechinids. 

 Moreover, the former have only twenty rows of plates in 

 the test, whereas this number is exceeded in the latter. In 

 the Carboniferous Perischodomus there are two rows of am- 

 bulacral and five rows of interambulacral plates, and in the 

 nearly allied Lepidechinus, of the Devonian and Carboniferous, 

 there are as many as eleven rows of interambulacral plates. 

 The Carboniferous genus RhoecJiimcs is very nearly allied to 

 the two preceding, but the interambulacral plates have no 

 primary tubercles. Lastly, in the Carboniferous Lepidestlies 

 we have another flexible Perischoechinid, with imbricated 

 plates, but the ambulacra were composed of no less than 

 ten rows of plates, and the interambulacral areas are 

 comparatively narrow, and are composed of several rows 

 of plates. 



IV. IRREGULAR ECHINOIDS (Echinoidea exocyclica). In this 

 section of the Echinoidea the test is generally of an oblong, 



