12 HAWAIIAN AND OTHER PACIFIC ECHINI. 



cases, in the Scutellidae, it is not far from the mouth and in man}- genera its 

 distance from the margin is an important specific character. Usually the peri- 

 proctal plates carry miliary spines and sometimes pedicellariae, but in some 

 species they are quite naked. 



The genital pores vary in number, size, and position. In Clypeaster and 

 some other genera, there are typically five pores and any other number is very 

 rare even as an individual variant. These five pores are in the interradii at. 

 or very near, the margin of the fused oculogenito-madreporic body. In the 

 Laganidae, the number of pores may be either four, five, or six and they may lie 

 far down in the interradii instead of near the abactinal system. When six pores 

 are present, there are two in the posterior interradius. In the other families 

 the number of pores is commonly four but five is characteristic of some genera. 

 The number and position of the ocular pores shows so little diversity that they 

 are of no use for systematic purposes. There are always five and the}* lie close 

 to the margin of the oculogenito-madreporic body. 



The madreporic pores are usually numerous and occupy most of the surface 

 of the plate formed by the fusion of the oculars and genitals; for this reason 

 I have called this plate the oculogenito-madreporic body. In the recent Fibu- 

 lariidae there is only a single madreporic pore and it is conspicuously large. 

 This is a useful systematic character and is of great help in distinguishing these 

 little clypeastroids from young Clypeasters, for the latter have at least several 

 madreporic pores even when only a few millimeters long. It should be noted 

 however that Fibularia nigeriae Hawkins, a Tertiary species, is said to have 

 numerous madreporic pores. 



In addition to the tubercles which bear spines and pedicellariae, the tests 

 of many clypeastroids bear low rounded elevations, known as ''glassy tubercles." 

 These take their name from the fact that they are composed of an unusually 

 dense carbonate of lime, which is clear and transparent and thus, when cleaned 

 from the overlying epidermis, resembles glass. The function of these tubercles 

 is not known and their use for systematic purposes is very slight. 



The buccal membrane in clypeastroids is generally thin and lacks calca- 

 reous matter but Aracbnoides is exceptional in that there are distinct plates on 

 the membrane, which carry miliary spines. The same Beems to be true of Ro- 

 tula, though the available material of this genus is too poor for a satisfactory 

 determination of the point. 



