108 ANNULOIDA. 



two genera are known in the Palaeozoic series, viz., Archao- 

 cidaris and Falcechinus, but these differ from all the modern 

 forms in the fact that the corona of the test was composed of 

 more than twenty rows of plates. The test was divided into 

 five ambulacral and five interambulacral areas, and the in- 

 crease in the number of plates arises from the fact that each 

 interambulacral area consists of three, five, or more rows of 

 plates (fig. 64). From this peculiarity the Palaeozoic urchins 



Fig. 64. Archaocidaris elliptic. The left-hand figure shows a portion of an 

 ambulacral area enlarged. The right-hand figure exhibits a single plate. 



were placed in a separate sub-order by M'Coy, under the 

 name of Pcrischoechinida (the Tesselata of Pomel). The five 

 interambulacral areas are surmounted dorsally by the genital 

 plates, which are five in number, and in Pal&chinus are doubly 

 perforated. The five ambulacral areas are " simple," or are 

 continuous from pole to pole, being surmounted by the ocular 

 plates. These are said by Baily to be triply perforated in 

 Palcechinus, but they are said by De Koninck to be wanting. 

 The test in both genera appears to be " regular." 



As regards the distribution of the Echinoidea in time, the 

 genera Archaocidaris and Paleechimts (the Perischoechinida) are 

 exclusively Palaeozoic, the former being confined to the Devo- 

 nian, Carboniferous Limestone, and Permian, whilst the latter 

 occurs both in the Carboniferous and in the Upper Silurian. 

 The normal Echinoids abound in the Mesozoic series, espe- 

 cially in the Oolitic and Cretaceous Rocks, and are also well 

 represented in the Tertiary Rocks. Their distribution will be 

 shortly noticed under the head of each family. 



The Echinoidea are divided into the following more im- 

 portant families, with their leading characters, distribution in 

 time, and a few illustrative genera : - 



i. Ananchytida. Mouth excentric, in front; anus behind, 



