EUECHINOIDEA. 



3/7 



the test is spheroidal, the ambulacra are only two-rowed, and the inter- 

 ambulacra are wide and are composed of three or more rows of plates. 

 The interambulacral plates (fig. 248, b) carry each a large perforated 

 tubercle, surrounded by a ring, supporting a long echinulate spine id). 

 As shown by Mr John Young, the test must have possessed a certain 

 degree of flexibility, as the edges of some of the interambulacral plates 

 are bevelled off. The Devonian and Carboniferous genus Lepidechinns 



A 



gjftb 





Y 



a> 



Fig. 248. — Arch&ocidaris Wortheni, from the Carboniferous Limestone of North America. 

 a, Fragment of the under side, with the teeth, of the natural size ; b, An interambulacral plate, 

 viewed from above and sideways, showing the hexagonal form, and the spine-bearing tubercle ; 

 c. Portion of an ambulacrum, enlarged : d, One of the spines, of the natural size. (Copied from 

 Zittel— after Hall.) 



differs from the preceding in the fact that the plates of the test are 

 strongly imbricated, the shell thus becoming quite flexible. The ambu- 

 lacral areas are two-rowed, but the interambulacra have from nine to 

 eleven rows of plates. 



Differing in some respects from the preceding are the genera Perisclw- 

 domus, Rhoechinns, and Lepidocentrus, of which the two first are Car- 

 boniferous, while the last is found in the Devonian rocks. In all these 

 genera, the test is rendered flexible by the overlapping of the inter- 

 ambulacral plates. 



Lastly, the Triassic genus Anaulocidaris, known by detached plates 

 and spines, is allied to Archtzocidaris, and, so far as known, is the latest 

 representative of the division of the Palechinoids. 



Order II. Euechixoidea. 



This division comprises all those Sea-urchins in which both the 

 ambulacral and i?iterambulacral areas a?'e constantly two-rowed, the 

 test thus consisting of twenty rows of plates. The genital and ocular 

 plates are almost always perforated by a single opening each ; and 

 a masticatory apparatus may be present or absent. The test may be 

 " regular " or " irregular." 



