PALECHINOIDEA. 



375 



The test in Bothrioddaris is spheroidal and " regular," the anus being 

 situated in the centre of the apical disc, and the ambulacra being 

 " perfect." The plates of the test are immovably connected, and the 

 interambulacral zones are composed of a single row of plates only, while 

 the ambulacral zones have two rows each (fig. 246, a). There are thus 

 fifteen meridional rows of plates in the test in all. The peristomial 



Fig. 246. — Bothriocidaris Pahlcni, Ordovician. «, The test viewed sideways, of the natural 

 size ; b, The apical disc and anus, enlarged ; c, The mouth, enlarged. (Copied from Zittel, after 

 Fr. Schmidt.) 



membrane (fig. 246, c) is furnished with triangular valvular plates sur- 

 rounding the mouth-opening, and similar plates encircle the aperture of 

 the anus {b). 



The sub-order of the Peri scho'echini dee comprises all the known 

 remaining types of the Palechinoids, and is characterised by the fact 

 that the test is composed of more than twenty meridional rows of 

 plates, and is " regular " in type, the anus being situated in the 

 centre of the apical disc. The interambulacral areas always possess 

 more than two rows of plates each, and the ambulacral areas likewise 

 may consist of more than two rows each (as in Melonites and OHgo- 

 fiorus, fig. 247). The mouth is placed in the centre of the lower 

 surface, and is furnished with a masticatory apparatus, while the 

 anus is situated at the summit of the test. The genital plates have 

 from two to five pores, and the ocular plates may have two pores 

 each or may be imperforate (fig. 245). Very commonly the test 

 becomes more or less flexible in consequence of the imbrication of 

 its component plates, the edges of which are often bevelled. All the 

 Perischo'echinidce are Palaeozoic, with the exception of the Triassic 

 genus Anaidocidaris. The earliest types are found in the Silurian, 

 but the group attains its maximum in the Carboniferous rocks. 



Of the genera of the Perischoechinidtz, Palcechimis possesses a 

 spheroidal test (fig. 244), the plates of which abut against one another 

 without any overlapping. The ambulacral areas are comparatively 

 narrow, of two rows only, each plate perforated by two pores. The 

 interambulacral areas are broad, and are composed of from four to eight 

 rows of plates. The apical disc (fig. 245, a) has five triply perforated 

 genital plates, and an equal number of doubly perforated ocular plates 

 (Baily), but the latter are wanting in P. sph<zricus. A single species of 



