EcMnoidea — Sea-urchins. 91 



living animal, are fixed strong pointed spines. In the centre of the Sea- 

 upper surface is the aperture of the anus, surrounded by a double °^° ^^'' 

 circle of plates (the "apical plates"), which have been regarded vill. 

 as equivalent to the radial and basal plates which form the most East Side. 

 essential constituents of the eup o-f the crinoids. The mouth is Wall-case* 

 a large opening on the lower surface ; it is surrounded by a girdle Table- 

 of five arches, each formed of a pair of auricles, which supports cases 

 au elaborate jaw-apparatus. 



Most of the recent Echinoidea are based on the same funda- 

 mental plan ; but it is often obscured by great changes in shape. 

 Thus, in the genus Galerites, though in all species the mouth is 

 central, in some the anus opens on the margin of the test, and 

 in others it is situated on the under surface. In other forms, 

 such as the common EcMnocorys scutatus (syn. Ananchytes ovatus) 

 and the living ''Heart-Urchin" {Spatangm purpureus), the mouth 

 is pushed forward near the anterior margin. In most of the 

 Echinoids, the spines are distributed fairly equally over the 

 test, but in some cases special forms af spines oceur along narrow 

 bands known as "fascioles." 



The test of an Echinoid therefore consists of the following 

 elements: the double circle of ''apical plates"; five pairs of rows 

 of " ambulacral," and the same number of " interambulacral " 

 or " interradial " plates; and in many forms also five pairs of 

 "auricles" and five teeth with their sockets. All bear spines, 

 some of which are modified into jaw-like claspers, " pedicellaria." 



The British fossil sea-urchins occupy Table-cases 76-78, and 

 the foreign Wall-cases 15-17. 



The oldest known British Echinoids are three species from the 

 Ludlow Series — Echinocystites pomum, E. uva, and FaUodiscus 

 ferox. These, however, are in an unsatisfactory condition of 

 preservation, and their structure is primitive. Specimens are 

 exhibited in Table-case 76. There are few representatives of 

 this class in the English Devonian, but a good series of species 

 occur in the Carboniferous. Of these, Palceechinm is the best- 

 known genus (Fig. 149). Like most of the Palaeozoic forms, it 

 has more than twenty zones of plates, there being three or more in 

 the interradii. Artlmocidaris is another interesting type, for the 

 plates are somewhat overlapping, so that the test may have been 

 flexible ; the teeth {e.g. specimen 75,940) are constructed on the 

 same type as those of the recent Cidaris. A polished slab of 



