Klem — Revision of the Palaeozoic Palae'echinoidea. 9 



was amply justified in considering the size of the spines of 

 specific importance. 



In connection with the ornamentation, must be considered 

 the imbrication of plates, which is also a very characteristic 

 feature. In the Melonitidae the imbrication is very slight, 

 while it is very marked in Lepidocentrus, Lepidechinus and 

 their allies. The imbrication of plates or the lack of it is a 

 very good feature for separation, as its presence or absence 

 gives the specimen a peculiar appearance. 



In concluding this general discussion, I wish to say a few 

 words about the genital and ocular plates. 



Among the Palaeozoic Echini the apical system of several 

 genera is known, showing very different types. In Echino- 

 cystis the apical disk is unknown. The madreporic plate is 

 separated from the apex and is inter ambulacral. The anus 

 is excentric and interambulacral, but near the apex. 



In Perischodomus, Perischocidaris and Palaechinus spheri- 

 cus, the oculars are wanting, the five genitals forming a closed 

 ring around the anus. In Perischocidaris there are sixteen 

 pores to each genital plate. In Perischodomus the genitals 

 are perforated by six or eight pores. 



In Bothriocidaris there are five large ocular plates around 

 the anus with small three-cornered pieces inserted between. 

 The apical system consists of ten plates and a ring of ten 

 smaller plates within these. 



In Palaechinus, Oligoporus, and Melonites the oculars and 

 genitals form a circle of ten pieces. In Palaechinus elegans 

 there are two rings of ten pieces each. Each genital plate is 

 pierced by three pores and each ocular by two pores. This 

 feature, however, is subject to great variations. In some 

 cases the oculars are without any trace of pores, and in others 

 there is a single pore in two of them. The genitals vary even 

 more than the oculars. In some three plates are pierced by 

 four pores, and the other two by five ; or three may have four 

 pores and the other two, three pores each. 



